I have a Kitchen Aide mixer, and a Cabelas grinder. I see they both can incorporate a tomato juicer.
Are either attachments worth owning? I have an insane amount of Roma tomatoes that need canned.
Anything else I should consider?

I just use a funnel shaped colander with the wooden pusher thing.

Seen one of those used a bunch.

May even still have 1 or 2 that I kept when we cleaned my grandpa's house out. Grandma was a canning fool.

No outside kitchen and no AC until they retired in the mid/late 80s.

Helped my mom using one when I was a kid. She canned and froze about everything. According to her, if we did not have at least a 100 quarts of green beans canned each year, we were going to starve to death. There were only 4 of us in the house....We had a BIG garden.
When I first got married, I showed my wife and mother-in-law how to can tomato juice.

There are about as many ways to process tomatoes for canning as there are ways to skin a cat. A long time ago, we used to put them in hot water long enough to loosen the skins, then cut them up and cook them a while, then run them through a strainer. That was too much work. We finally bought a hand crank juicer, then an electric powered that we currently use. It eliminates all that heating and cooking, just juice them, put in jars, and can.

Mom has a Squeezo bought in the eighties that works great.

The parent company was bought by a big capital firm, before I
bought mine in the 90s.

Ours looks just like Moms.

For some reason the aluminum in the body reacts to tomato acid
and turns the juice black. If you keep stuff going through it, don't stop
at all, you don't get any black. But you still get the weird taste.
It's a piece of junk.

I would strongly advise making sure the body of anything purchased
Is stainless if possible. I've seen plastic ones, no idea of durability.

I don't juice them, I just make sauce with them, so I use a food processor. Probably not helpful but there ya go