My thoughts about your first few points:

• I, and many others it seems, think the Sync for Reddit app was priced almost too low. Only a few dollars for an app isn’t sustainable for the dev in the long run. I believe people using reddit sync with ads actually supported the dev more than people buying the app.

• The reddit API was free, so it technically didn’t have ads if you used some ad-free (FOSS) 3rd party app. Sync for lemmy isn’t any different in that sense.

• Sure, the lemmy devs and instances also need money to run the service. People are free to and should donate to them as well. Sync for lemmy, however, isn’t costing instances any more than FOSS apps.

The rest of your points are just personal preference, really. If you like the app, buy it or use it with ads. If you don’t like the app, use something else. Honestly surprised by how much hate an app no one is forcing people to use is getting.