when you ask even for a small donation, you make people think about what they are spending their money on, and people don’t like to think about what they are spending their money on - it makes them depressed. it’s much easier when it’s presented in terms of a transaction to unlock or buy something, with the price at whatever people of a given region would be willing to spend without thinking. unfortunately that manipulative approach doesn’t fly in foss cause you’d just get forked in no time with everything unlocked, so you have to be fair, and as a dev the only actually valuable thing you have to sell is your ability to dev, which is a kind of service (not a product), so you’ve got to find a way to market that - make your process more transparent to the users and allow them to inflience it in some way and give extra privileges to donors or something and then pray to the pop culture gods that you become popular enough for any of that to matter
SaaS is a whole other deal, if you actually understand what it is you can do it in fdroid, and it would be just as bad as anywhere else, and your only punishment would be an anti-feature tag