Add a 2nd drive to time machine when one becomes full?

You can not merge or extend Time Machine backups across drives.

Get a second drive that is at least two times larger than all of your combined data and start new Time Machine backups on it. Then once you have all of the drives backed up to the new TM drive, you can discontinue using or erase and repurpose the smaller TM drive.

Well i would ensure to have a large drive, then …. Why would you keep 50 backups ?

i tend to clean oldest once in a while if TM does not seem to handle this often enough for me. I like to ensure there’s plenty of space

I used TM occasionally years ago but not any more.

Most data is pretty useless and a waste of time and space backing up.

All my essential stuff is copied onto other drives manually.

Also remember that TM will erase the oldest backup when it is full to make space so it will continue to backup.