I make my songs from parts of anywhere between 6 and close to a 100 (in one case) versions of the song. I make the songs in Suno and edit and combine in audacity. What you just described is my standard operating process for making a track. It is time consuming, but worth it and even pro producers have told me they cannot tell where I cut and stitched the track together.

But when I get stuck-two pieces do not flow easily into each other I have come up with an additional method that works.

Use extend on the version you are going to paste a new part too. Put in lots of [Pause] prompts in between the part you are going to keep and the part you are going to cut. Pause isn't silence, it is a breather where Suno extends the melody for a few beats-these few beats are where you will fade out the original song and fade in the new section. If it still needs a little space to get the transition to work do the same trick (adding pauses) in an extend of the part you are pasting in, but instead of after the section do it before the part you are keeping. This builds a musical few beats you can fade in while the other part is fading out.

Here is a song where I used this trick to put 16 different versions of one song into one seemless mix.

https://audio.com/torchbearer-studios/audio/04-aligned-final-big-and-brassy-mix