The problem here is: People give you tons of advice, but actually, no one has a clue wtf is going on. Nobody knows why Google's algorithm suddenly messed with your website. As you wrote by yourself: No manual penalty. No issues.
The same situation / algo penalty happened to a couple of my sites as well. I tried everything. Disavowing links, changing the structure, onsite and offsite optimization, audits, changed the hosting, etc. - nope, nothing had an impact.
And then you have all these wannabe SEOs and Google employees like Muller who just blame the webmaster "Oh something must be wrong with your site. Try to focus on high quality content, so we can steal it...hehe"...well, how about something is wrong with the algorithm?