Hm. If I remember correctly, these monitors I've got have both DVI and HDMI inputs and I can switch between those inputs on their front panel.

If I got something like this...
Thunderbolt 3 to Dual HDMI Adapter

Then I could theoretically connect the older gear with DVI cables and the mac with the HDMI cables and just switch between the inputs on the monitor front panels.

In theory.

Sad, though, that those dual-HDMI thunderbolt adapters don't also have a USB connection and a power charging pass through. At least I don't see one available. You either get a single HDMI and the USB and power charging pass through, or you get dual HDMI, not both. Does that seem to hold true?