I think the “hot take” that needs to be said here - and surprisingly I have not come across it yet - is that it’s totally fine if you’re willing to pay 20 dollars for the app.
However, please remember that the app itself is not self contained. It relies on the hard work of the Lemmy developers and the instance you use staying online. Please consider sending them some money as well down the line. If you get the app for 20 dollars as a one time payment, maybe some future payments - even if considerably smaller - can go towards the latter.
I did the opposite. The moment I saw Sync for Lemmy available to download I installed it and immediately purchased Ultra because bless LJ for making the superior app. He deserves getting paid for his work. And it's not like he requires you to pay for the app. It's free for everyone, you just get some additional features if you are willing to pay and by doing that you also support the developer. So I decided to support him. I've been using Sync for Reddit before and I knew I needed this app for Lemmy. Now I feel home.