For sure I can recommend Inoreader but I use it for RSS - I don’t know more powerful service. But newsletters? I use ordinary mail reader Spark. I prefer RSS for sure where I have e.g. 200 news every day. Newsletters - I avoid such subscriptions. For sure Spark has “smart folders” which could help a bit.
Two of the recent answers are what I do. I use Reeder which is inexpensive along with Kill The Newsletter. I’ll use the direct RSS feed like Substack’s if it has one.
One benefit of KTN is that if you want to take a break from a newsletter, you just delete it from Reeder. You can then add it again without having to resubscribe to the newsletter itself.
Using KTN also keeps you anonymous. But most importantly, keeps your email inbox a lot more sane. You get to choose when you open Reeder and look at the newsletters vs the constant pings in your inbox.
Tip: if you go the KTN route, write down each feed so you have it.
FYI since someone alluded to it. If you use Readwise Reader, unless they have changed, I’m pretty certain they pull feeds on a schedule so you may not get an email as soon as you would like.
My idea is simpler, I setup an email rule that directs all newsletters to another folder called Feed. This way they don’t fill up my Inbox, but they are sitting there for me when I have time to read them.