Advice on accessing an old mac pro without a monitor. Help!

Hi all.

I have a mac studio and studio display, and have a couple of old mac pro's in storage, ones a G5, I think it was a 4.1 with various upgrades. the other is even older.

I'm finally going to let go of them. Last I used them they were fine, but now I have no monitors they are compatible with, so I can't erase any personal data on them before sending them on their way.

What do I do to make it safe to give them away?

Now, my first thought is to use apple remote desktop, but how can I do that without first enabling it on the old machine? And since the old machine has obsolete information about the local wifi network, it won't even log on to the network. Is there a usb or ethernet method of getting into that old machine? Or how else can I make it safe to give away?

Before I go down a week long rabbit hole of trial and error, figured I would ask the experts for a possible game plan that didn't involve buying a bunch of stuff. Thanks!