One can mount a bike either by holding the bars and throwing the leg over the seat, or holding the seat and throwing the leg over the bars (particularly good if doing heavy touring).

How low of a step through do you need to go?

If one hunts, one can find a vintage step through made with Reynolds 531 tubing, some Columbus tubing, or some Vitus tubing. Mom has a Motobecane Grand Jubilee Mixte which was nice for the day. But I've seen other vintage manufacturers too.

For newer frames, there is the Soma Buena Vista.

Rivendell makes the Clem Smith Jr. I don't know much about it. It appears to be a fairly heavy duty "touring bike". There is one for sale here in Eugene, but absolutely HUGE.

If you are going to get S&S Couplers installed, it may be beneficial to find a bike with two main tubes rather than three.

I don't know quite what I'd hunt for. There are a couple of Motobecane Grand Touring Mixte frames on Ebay. One step below the elusive Grand Jubilee.

https://www.ebay.com/itm/255630142908

https://www.ebay.com/itm/255733265498

Here is a Buena Vista which is fairly new.
https://www.ebay.com/itm/204110485506