You’re not paying to remove ads from Lemmy. You can continue using Lemmy ad-free on mobile via the mobile site or any of the other PWA’s or native apps. What you’re paying to remove ads from is Sync. The developer has decided that they need to be compensated to sustain the amount of effort developing and maintaining the app requires. If you don’t want to pay that price with cash or your eyeballs then don’t use it.
Nobody is forcing you to use Sync, nobody is forcing you to see ads. The beauty of a platform like Lemmy is you have the choice to use whatever client you want. That doesn’t mean you’re entitled to any of them.