It might be a good idea to hide the button until a user scrolls up a little first. This way the users intent is clear, they either want to scroll up a little or a lot.
Thunder App
An open-source, cross-platform Lemmy client for iOS and Android.
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Great idea!
The floating button is currently being overhauled which I think should make changing the position easier even if it doesn't support it out of the gate.
In the mean time, you could turn off the floating button for the feed view and instead use the feed button on the bottom navigation bar to scroll back to top. Bit of a hidden functionality :) Don't think there is currently a work around for the post view though unfortunately.