I'd personally like to avoid KYC just because I'm expecting every website will inevitably have a data breach
While you are correct, my current opinion on the topic is that it doesn't matter. You're not doing anything illegal or immoral by buying crypto, so I don't even try to advocate to people to avoid KYC to buy Monero, even if they intend on doing slightly illegal things with it, once you're in the Monero ecosystem you can't be tracked anyway.
If you mostly worry about fees, then you'd have to look in details at Haveno Reto and what offers there are, some are great, but in some parts of the world and with some payment methods, usually you'll get a penalty compared to market rate. In most cases it's more economic to keep it KYC and buy Monero directly on Kraken, or buy LTC on Kraken then swap it to Monero.
If you still want to buy with as little KYC as you can, then yes, you can buy crypto on Cashapp (I recommend Litecoin, you'll save a bit of money with the transaction fees, and it's faster). If you want to use it for Haveno deposit, you'll need 0.11 XMR minimum (20 dollars or so), so you'll need to swap your LTC/BTC to XMR in the first place, I too recommend Trocador for that.
Concerning wallets and OS for OpSec, there are two tiers as far as I'm concerned:
1 - You do not think the police raiding you and doing computer forensics on your devices is a possibility, in which case you shouldn't care about that stuff, just take whatever trusted wallet, no need to run your own node, and no need to use a special OS, windows or android is fine.
2 - You think the police raiding you and doing computer forensics is a possibility because you're a big political agent, a big tax fraudster, or a drug seller, then you will need to use Tails/Whonix. You can read about all the paranoia and precise OpSec done for those that are at risk on the Darknet Bible that I attached to this post. If you want to read more about advanced opsec I recommend browsing/posting on Dread.
Other than that you're pretty good to go my man. The guides you link are reputable and your plan is sound. You're more prepared than 99% of people buying Monero and trying to use Haveno. If you want to see how cavalier people can be about it, browse r/monero and r/darknet, even those retards manage to get Monero after doing halfhearted attempts.
And if you're a schizo that doesn't even want to buy Bitcoin with Cashapp because you feel like it could be used as an entry point for the police to interrogate you about where it went and what you did with it, I can send you the 0.11 XMR you need for deposit, I trust you'll send them back to me after you do your transactions, I don't mind.