Tips for tempo management?

I’m currently working on a new song that features plenty of gradual tempo changes. Does anyone here have any information on how to gradually change tempos without desyncing the music from the added vocals?

I don’t think SynthV can adjust the tempo of your .wav file. Your vocals must match to the backing track.

I use Reaper for stuff like that. Set the grid in Reaper to match the backing track. Export a MIDI (it contains the tempo information). Import that MIDI into SynthV.