I think the trick is to not let the perfect be the enemy of the good. I run GOS and have apps from fdroid, obtainium, aurora and play store. I used to have google services in the owner profile but recently moved all my apps that require google services into the private space so my main user space is fully degoogled.

The nice thing about GOS is that while they take a hard-line stance on security the OS itself is fairly agnostic to what you do. You can also change your mind over time about what you are ok with, like I did. Privacy, security and anonymity are all different goals to strive for but its always a a balance. You are already ahead of the curve just for trying. Keep it up!

I'm nearly fully FLOSS. GOS recommendations are geared for political dissidents and other actively persecuted persons. Take your threat model into account. Most people don't have that kind of worry.

I've been using grapheneos for two or so years, I think using some non-foss apps is probably non-negotiable if you want a functioning phone. I personally only use one profile (+ the private space), but separating stuff out isn't a terrible idea. Knowing that play services are sandboxxed away and that I have much finer control over per-app permissions makes me less concerned about letting corpo stuff cohabitate.

However, I recently learned that GOS recommends Play Store before any FOSS app stores.

As for this, basically the gos devs are just kinda indiosyncratic and have a lot of opinions, I wouldn't worry too much about it if you know what you're doing. I use fdroid for most stuff and I've never had any issues.