I think now is a great time to remind everyone, like sync's developer, Lemmy's developers need to be paid too! The amount of time all the devs put into making lemmy exist, in my opinion, should be worth some of your money. If you can afford it, donating to the people who develop lemmy and/or the people keeping your home instance up will accelerate the incredible growth of lemmy!
Personally, I think the app is absolutely fine without paying for the subscription. I really don't see too many ads. I get an average of one ad for every two refreshes of the feed when scrolling through, and none of them have been obtrusive. It's certainly night and day when compared to the ads in the official Reddit app.
I mean it's just a port of the Reddit version that was made by the same dev and had been around for over a decade (with constant updates and improvements), I don't think enshittification is a given.
Well, compared to Reddit getting filled with "ad looking like real post", and "ad written as a comment", and "mass downvote everyone except the ad"... it's really hard to stoop that low.
What is likely to keep Sync in check, are the alternative apps. Reddit got enshittified because it became the single point of failure.
Depends on if their business model works. If enough people are paying for premium and using the free version of the app to pay for development, there is no reason for them to increase the amount of ads. In fact, if they do show too many ads, it will likely push a lot of users to alternate apps.