FFXIV has a pretty strong wall between normal content and high-end content. If you're casual and your endgame mostly consists of grinding out your tome gear, relics, crafters, gatherers, hunts, mounts, mogs, titles etc then there's not really much toxicity to encounter. If you go into the high-end (extreme, savage, and ultimate), you'll probably experience more of it. The sweats are well-conditioned to keep their autism to the high-end because they know (consciously and unconsciously) that they need the casuals to stick around in order to give their raiding achievements meaning.
In terms of difficulty, Shadowbringers and Endwalker will probably feel like a noticeable jump in difficulty just because those expacs are what the game's current systems are 'tuned' around (you probably experienced this moving from HW to SB). That being said, if you manage expectations (typing "Hey everyone, new to this instance" when you first load in), you probably won't experience any toxicity. Even 'tough' normal-mode content in ShB and EW is designed to be easily completed by randos in 1-2 wipes at worst.
As for the leveling grind, here are my suggestions in no particular order:
1) Run your best daily roulettes every day - leveling, main scenario, trials, alliance raids, and frontlines. These have the best xp rewards for your time.
2) Eat your food. All food gives the same +3% XP buff regardless of value so even cheap 1 gil food is fine.
3) Join a free company. Most FCs run the various Heat of Battle buffs which boost experience gain.
4) Under level 50, complete each guildhest at least once on each of the ARR classes. The first-time completion bonus is very generous and these are very quick to run.
5) Palace of the Dead and Heaven on High are highly efficient xp-wise but they're soul-crushing to grind unless you have terminal autism.
6) Highest-level dungeon you can run that isn't an endgame dungeon (so dungeons that are not level 50, 60, 70, or 80) are the most efficient way to level once you're out of roulettes.
7) Daily hunts can give you almost whole level. There are HW hunts (50-60), SB hunts (60-70), ShB hunts (70-80) and EW hunts (80-90). I like to do these while waiting on my queues or if I just need a break from dungeon runs and want some chill content to do while watching a movie or something.

Beast tribes. These are repeatable daily quests for reputation that you might be familiar with if you've played other MMOs (especially WoW). HW has the Vath and the Vanu Vanu, SB has the Ananta and Kojin, Shadowbringers has the Pixies, and Endwalker will have the Arkasodora in a few weeks. These quests will level sync to any level in their xpac's range (so stormblood quests will sync anywhere from level 60 to 70) and give a lot of XP as well as the reps gating glamours and mounts. There are also beast tribes for crafters and gatherers but leveling those is an entirely different discussion (mostly revolving around how much gil you're willing to spend).