I'd like to setup my new mac from scratch, but still backup to my existing time machine backups. possible?

I'm not quite sure I completely understand what you want to do, but if you hold an option key while selecting the Time Machine icon in your Mac's menu bar, a selection "Browse Other Backup Disks..." will appear. Perhaps that will accomplish what you seek.

Having said that, all my new Macs since forever have had their contents populated from the ones they replaced. That goes back before macOS was known as macOS, even before OS X, which is getting to be 20+ years by now. When you initially set up a new Mac it will ask you if you want to transfer contents from another Mac or its Time Machine backup. It doesn't really matter which of those two options you choose, but that's what I do, and what I recommend. The transition will be as seamless as practicable.

Of course if you want to forego that option that's ok too. The older one's TM backups will remain unaltered and available for you to use.

Set up your new Mac - Apple Support

IIRC, i can probably back up to the same external TM disk(s) that my current machine does, but the new machine's backups won't overwrite the other backups.

That's correct.

I just prefer to delete old / unused files manually. 32 bit apps simply won't load in Catalina. They occupy space, but that's all. You can delete those things at your leisure. If I come across a file dated sometime in the last century I probably don't need it. There really aren't that many places for things to hide, besides, macOS's recent "reduce clutter" option makes it easy to find old and unused files.