Personally I'd recommend picking an instance that at least has Hexbear and Lemmygrad blocked, as that removes 95% of the toxicity you'd see on the platform. You can check what an instance blocks by scrolling to the bottom of their main page and clicking the instances link.
Sopuli.xyz is a really solid medium sized general instance, with good admins and an updated lemmy.
Lemmy.cafe is a nice small instance (not much local activity, so you'll need to rely on the All view and your subscriptions for that one).
If you have affinity with Anarchism (or environmentalism or sailing the high seas), Slrpnk.net and lemmy.dbzer0 are both excellent.
But if you'd like a more themed instance that appeals to a particular interest, by all means go for that instead. https://lemmyverse.net/ or the instance finder tool at https://join-lemmy.org/ are good places to find one. :)
Unfortunately an individual blocking an instance only stops posts from those instances from showing up, but you can still see comments and responses from the users of the blocked instance.
If by leftist, do you mean actually leftist, or do you mean that you think the profits from imperialism should trickle down to white people in the imperial core?
The leftist instances are: lemmygrad.mlhexbear.net and to a lesser extent lemmy.ml but the latter is federated with more instances
When I first joined Lemmy, I made the same mistake.
I'm now on lemm.ee, which is quite a tolerant instance with none of the bullshit... at least not yet. .sdf is also a good instance (I have that as a backup account).
I think the only things that matter are connectivity and defederation. As long as you have a continuous healthy connection to them (no lag, etc) and they don't defederate from the rest of the fediverse you should be ok.
hexbear is objectively the best instance once you learn how to not be a liberal on there. best community, best vibes, best memes
I notice you've gotten suggestions to steer away from them because of toxicity. They're toxic to transphobes and imperialists. People who consider themselves on the left get absolutely enraged at being criticized morally from the left. Look behind anyone complaining about them and you'll realize they said some wild shit and got rightfully scorned.