The first community I missed on reddit after they stayed dark was ich_iel, and they moved to feddit.de.
To be honest, I kinda regret that choice, and I usually browse kbin.social. Smaller instances fail to provide the varied experience that made reddit great for me.
I did also start out on feddit.de but soon switched over to Lemmy.world due to too many limits when it comes to federation (e.g. general ban on nsfw instances).
Since I can still participate in all the communities on feddit.de this does work great for me
You find a post on <foreign instance> (because the feed on your small home instance is very limited) and just want to upvote it.
You get to the login screen and remember that this isn’t your home instance.
You open your home instance, navigate to the search form.
You try to find the link for the original post that you can enter in hour home instance’s search bar (which isn’t at all easy to find if you don’t know where to look).
You paste the link in your home instance’s search form.
You open the post on your home instance.
You vote.
This is frankly unacceptably complicated for a regular, casual user to just cast a vote on a post.
Honestly once you are subscribed to an instance I don't think it is a hassle.
If you find the post in the feed of your home instance (for example on All) you will directly post via you home instance. If you are subscribed you can access any foreign community via the communities tab of your home instance.
So at least in my experience with Lemmy I never had the issues you described.