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.

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

This is where Firefox Containers come into play. Isolate the worst offers like Google, Amazon and Facebook into their own containers (one each), then the rest into another one or multiple. I keep a Shopping (Amazon), Personal (free Google), Work (paid Google), Facebook (all Meta apps), and a few others. I also use a cookie manager that is container aware in case I need to sanitize one container but not the others.

One thing nobody told me and really messed me up when scheduling meetings is that Librewolf sets your timezone to UTC.

Sounds like you already have a pretty good plan in place.

One extension you might consider is libredirect. Instead of manually copying a random youtube link, pasting into the url bar, and changing youtube to yewtu.be, you can just click the link and let libredirect do it for you. If the front-end gets 429'd, you just click the "switch instance" button until you find one that's unblocked.

You might also like Simple Tab Groups, if you ever have trouble compartmentalizing a million open tabs. Might be disruptive to your workflow though.

As has been suggested, Containers are excellent at keeping aggressive local storage isolated. Create a Container for each of your problematic must-haves.

As well as the two add-ons you already have, I'd recommend LocalCDN.

The only issue I've had using LW for a number of years is it struggles with CAPTCHAs but thats a small price to pay really.

A practical advise. A lot of websites refuse to open on Librewolf because of an OSCP related error, a lot. You can disable that in config if you don't want to scratch your head every time.