Adding a New Machine to a Muli-machine Setup

Hello Apple Community,

I'm a Creative Director working on two separate iMacs side by side- one of which is getting up there in years and now having some unpredictable software issues. I've done the work, gone through reinstalling the OS, tried to limit the task it needs to compute, etc, but unfortunately it's a machine that can't be updated beyond Big Sur and I feel like needs to be put to pasture at this point. Thankfully I have a third machine I can integrate.

The business I was working for was sold last year, and I thankfully was able to bring my "office" machine home with me as sort of a severance deal. If you look at the below image, you'll see that I've listed my three machines and their hardware information. What I would like to do, is take all the data off the "new" machine, and have it accessible from an external drive. It's a lot of old files that I don't need to use ever day but I don't want to completely delete them either.

Then take everything from my "alpha" imac and transfer it to that "new" machine so it will be my new main machine. The "alpha" will then take the place of my "beta" computer and will run my email(s), internet searching, music streaming, etc, tasks. I'm not sure if I want to take the data from the beta machine and have it accessible from an external drive or transfer it like the original "alpha" iMac, however seeing that it's running Big Sur that might be an issue... If that won't work I can always wipe the current "alpha" and start anew.

What I need guidance on is how to properly go about all this without irretrievably deleting data and without being down a few days. Ideally I'll end up with two functioning iMacs and all the information from the one/two on an external drive that I can access when I need to.

Would it work to take all the data from the "new" machine and the old "beta" machine and put it on one external drive that will be accessible to my new configuration?

I don't feel like I'll be able to rely on the "beta" machine much longer, so any help would be appreciated. If you need any other information regarding my systems please let me know.

Thank you,

/t