I have an account and I love the product.

But the big drawback is that you can't really search full-text in the body of emails because of encryption on server side. And searching in the subject is most often not enough.

I really wonder how people deal with that.

If you don't have many emails in your archive, you can use their web storage full-text search, but that's not scalable.
You can also use Proton Bridge with Thunderbird to import your emails locally and search full-text, but that's not handy through multiple desktops and mobile.
So you're left with over-engineering your email subjects, labeling and folders, which is not manageable.

I'd love to give up on Gmail and go 100% ProtonMail but I need to solve that problem first. 

 I think that is the tradeoff between encryption and searchability. When I need to search something across my e-mails I use Thunderbird with proton bridge on my laptop. 

 the tradeoff seems obvious at first in favor of privacy. But when I consider that I need to search quite regularly (sometimes on mobile) in thousands of emails of at least the last decade, the tradeoff is not easy anymore. 

 https://proton.me/support/search

While not ideal it can be enabled in web app if your browser allows web storage.  

See this article.  

 thanks. yes, I've already tried that. It works with a reasonably small collection of emails, but with an archive over 5Gb it's not really a workable solution. 

 I assume increasing the cache size doesn't help? 

 I've not tried that and I don't really want to be tinkering with browsers everywhere (I use 2 desktops, 2 laptops and 1 smartphone). I need something easy enough.

I initially hoped that ProtonMail = Gmail + privacy but in its current state and with a big archive, it's ProtonMail = Gmail + privacy - searchability, which is frankly less compelling.

So far, I've moved all communications with banks, shopping sites and private conversations to Proton, but Gmail remains important for the rest and I don't find it handy to need managing both.