No, then you won’t even be able to use in-app purchases.

That's not true - they wouldn't be able to use the Google Play APIs for payments of course, but if the app is sideloaded they can definitely use any payment processor / method. If the app isn't on the Play Store then Google has no say over it.

Android supposedly has an option to side load, and even install another store, but in order to do it, you get through a series of warnings

It's really not as difficult as you make it seem.

1. Send a link to the user somehow (SMS, email etc); or user goes to the website
2. Click on the Download button
3. Open the APK
4. In the dialog box that pops up, click on the Settings button > then allow Samsung Internet
5. Click on the Install button

That's it. There were no "series of warnings" to go thru, no need to flip between multiple screens or anything. I literally just went thru this process to install the Epic store my Galaxy Fold 4 - which took only a few seconds in total - and it was in no way complicated or "scary" at all. And bear in mind that the audience in this case are gamers - people who are already familiar with the concept of downloading and installing programs on a PC, so it's not like you're targeting some tech-illiterate people here.

The only one that lasted is F-Droid

Not true again. Aurora Droid and Droid-ify are both reasonably popular, at least in the OSS/enthusiast communities. Yes they use the F-Droid repos but they also subscribe to other repos (Guardian Project, Izzy etc), so you're getting your apps from multiple sources.

There are also proprietary stores such as Aptoide which are quite popular in the Asian markets. Finally, you're completely ignoring other stores which are bundled out-of-the-box on many non-Google phones such as the Galaxy Store on Samsungs, Mi Store on Xiaomis, AppGallery on Huawei etc. Of course, in the western market the Play Store is the most dominant, but the Samsung store is reasonably popular among Samsung users (as they have regular deals on games and various other apps + some exclusives like Good Lock and other Samsung-specific apps), and of course, the OEM stores are also quite popular in Asian markets.