I have some accounts saved in my previous browser e.g. Google, Discord, Spotify etc. that I don't use really much, but require them regardless

I personally keep an entirely separate browser profile for Google, and another separate profile for Discord. Just further isolates things even more than Firefox/Librewolf already does, plus extensions and tweaks and such can be customized as needed per profile, etc. Profiles can be accessed through about:profiles in the browser FYI.

I use uBlock Origin and SponsorBlock as the only extensions

That's pretty good. The only other extension I'd recommend is Skip Redirect, its pretty useful and improves privacy, also recommended by Arkenfox.

If you want a more private way to watch YouTube as well, you can try Piped. It proxies all connections to Google/YouTube (so never directly connects to them and their servers), plus removes ads, tracking, and telemetry. It also has SponsorBlock, Return YouTube Dislike, and more built-in, so would make the SponsorBlock extension unnecessary too. Piped does have its downsides, but I'd recommend trying it and seeing what you think.

If Piped isn't for you, my recommendation is to just watch YouTube either logged out, or signed in with my suggestion above of using a separate profile if you really have to.

I installed Mull in my phone in the hope of pairing it with Librewolf while avoiding creating a Firefox account if possible

You will need a Firefox account for syncing. The good thing is the data synced is encrypted so not even Mozilla can see it, and Mozilla does generally have a good reputation with things like this, so I don't think its a major concern.

Another general tip I can give is to set the browser to clear cookies and site data on close, and just make exceptions for sites you need to stay logged in to or set preferences on. This helps massively in preventing sites from tracking you and when, if, and how you use them, it also prevents unnecessary storage space being taken up on your drive, etc.