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I have no idea how to make this more clear, not to say it was clear to begin with because even I don't know exactly what I want. Literally just, anything. I don't care. Just throw shit at me and see what sticks.
Best thing I can think of is play a couple RPGs that are really different (i.e. not all minor variation on D&D) and think about what will work for what you want to accomplish and what won't. The most important thing is to identify what you want to do in non-technical terms before worrying about probabilities and level-up schedules.
The best thing to do is to read a bunch of RPGs and find what you like, what works, what doesn't work, what's missing?
Suffer not the furry to live, but the Wolfaboos need to be the first against the wall.
For what it's worth, I think the concept of a Warlock "talking to" their patron is a total misread on the intent of the class in the first place. When I first read 5e, the Warlock was one of the things I really liked as it seemed like an actually new and fun class with a cool Dresden Files kinda flavor. A patron has to be something capable of granting 20 class levels worth of power while not quite being a traditional god/goddess. This opens things up to epic-level angels, princes of Hell, archfey, Cthulian outer gods type entities, etc as spell-granters instead of just making them Cleric options. The Warlock should start out mostly in the dark, receiving only an infinitesimal trickle of this massive power from somewhere they can only guess. As they become stronger, they become capable of using more power and knowing more about their patron. This idea of "my warlock calls up Jimmy the two-dicked satyr, his patron, to have a chat" is gay and retarded because the kind of being that would just talk to a low-level nobody wouldn't be capable of being a patron in the first place.
You do have an idea how to make this more clear because I spelled out the distinctions of what you might mean in my reply. Are you looking to write a rules system, a setting and adventures or both? Saying "I want to create a TTRPG" implies both but your question of asking what resources you need suggests you want to create a setting and adventures. It implies it because if you want to create a rules set your resources for that are: pencil, paper, looking at other RPGs for inspiration based on the sort of game you want to create.
This not only requires him to be coherent enough to think through this and plan, but it also requires the collective actions of every single deity, demon, or devil, of every single alignment, in every single setting mind you to miss this. I should note that in most incarnations he's not capable of organized thought himself any more due to his attempted transcendence and efforts to wipe out all other gods but himself driving him completely mad by the way, but this also pisses in the face of pretty much every single edition prior to 5e.
Give GURPS a try. It's janky as hell, but right out of the box it can be used for any setting you can imagine. The myriad of supplemental books, both first and third party, refines it for a more custom experience. I've played traditional sword and sorcery games, a historic reenactment of the First Crusade, a zombie apocalypse, a custom Fallout campaign, space pirates, cryptid hunting in the 1960's, a noir detective story in the 1930's, and I'm working on a super hero game.
The "fuck the rules, roleplay as whatever you want" attitude that permeates modern RPGs fucks with Warlocks as much (if not more so) than Clerics and Paladins. A Warlock's Patron should heavily affect them, either due to being chosen for their natural affinity with said Patron, or because you're quietly scared shitless of what happens when your debt comes due and you have no idea why your Patron is telling you to do the things you're doing.
But why though? Even the fighter who learned to be good at 2h weapons can have someone that trained him that turned out to be bad guy and isn't necessarily the big bad evil guy but could be one of their lieutenants, or maybe they went on a training expedition and disappeared or went after an artifact, or maybe got defeated and needs avenged, involving something that happens to also be a part of your story and the party encounters a city where the previous captain of the guard went missing and enough hints point out to the fighter who it was.
I'm gonna keep it real, I don't know Faerun lore, my group has never been FR guys. If there are other lore reasons that it would be impossible for Tharizdun to be a patron, I default to your knowledge. I just wanted to point out that a Warlock can most certainly have a horrifying, inhuman, outer god kinda patron because the idea of the Warlock just having telephone conversations with their patron is simply a dumb misconception of how the class should work (presuming one cares about the class working in the game and not just existing to provide stats.)
I think it's a bit silly to blame the rules for not having binding passages on how Warlocks behave or the lore effects/consequences of their powers. The DM should be able to decide on and adjudicate that on a case by case basis. If I were DMing a game where someone wanted to be this kind of Warlock, I would say clearly "By a certain level, you will basically be King in Yellowing the average person you encounter and that's going to create problems. Are you and the party up for that?"
Eh, Warlock pacts aren't so burdensome on game lore, not any more than Paladin oaths or Cleric domains are. They're also not particularly difficult to reflavor (provided the setting has magic of just about any kind). It's stated in the PHB (iirc) that some patrons have a bunch of Warlocks just to maintain some foothold among mortals. The patron may tug on the chain every once in a while, but there's no need for the Warlock to become the super-special MC of the group. I've found that if a "Main Character" emerges in one of my parties, running or playing, it has a lot more to do with who wrote the best character hook and roleplays the best for that adventure, rather than anything to do with class.
It doesn't have to be in the PHB. But if I as a player show up at the table and my character may as well have not existed in the world until the start of the adventure, that's not a character that has much going on in the world let alone a reason to be doing anything. I would hope your players aren't so basic that the only reason to ever go adventuring is "because its' the right thing to do" because that's one-dimensional as fuck.
Your table sounds boring. If you can't figure out how to keep a player engaged, that's a you problem. No, it doesn't mean that character backstories need to "take over" your campaign, it means that people come up with characters that exist in the world you've created and expect that the PC doesn't exist for the adventure as if you put them there as a generic fill in character creator from a CRPG to be run through a dungeon meatgrinder like you're playing Chainmail.
And if that happens and things aren't balanced because one player is hogging the spotlight, you tug on another hook from a different PC's character sheet to balance things back out.
To clarify, if an MC emerges it's usually via an unspoken party consensus because everyone enjoys the hook and the roleplaying. Nothing wrong with enjoying playing a simpler character or a character who primarily supports one of the other PCs. Few things worse than a party where everyone is competing to be the coolest.
Certainly, I wasn't referring to a party where everyone has a case of main character syndrome. I simply meant that if a PC's former sensei or whatever the shit has been coming up too often, move onto the guy who taught the rogue sleight of hand tricks when they were a kid and tries to sucker him and the party into doing a job. Doesn't mean the rogue needs to engage with it like it's some holy grail of a quest, but it can keep interacting flowing around the table smoother or even to get the party back towards the main story/quest/adventure.
