I have had this problem while buying stuff.
I have made a lot of research and I have contacted google support multiple times about this.
They added this “new feature” as an extra layer of security. Of cource it has some problems. But I think they also made it to stop people selling stuff through google services.
There is no method to avoid this. BUT. After the phonenumber change it will start the clock.
For example in YouTube channels when you sell. You have to waut that 24 hours to give the full permission to someone else correct?
This works like the same way. I sadly don’t remember how long it took. (After this added I only bought 2 more accounts which included gmail) But after a while it changes. After that they only ask recovery code from the new phonenumber.
I bought Clash Of Clans account and it had this problem. Seller sent me the codes always when I needed them. Then at one point it stopped to ask the codes from his phone and it started to use my phonenumber. I told him that it finally successfully changed and we closed the deal.
Hopefully this helped ya guys even a bit
It was around 2-4 days