If well made they should hold up. I did a google search and found this link:
A Complete List of Womens Touring Bikes: Step Through & Mixte - CyclingAbout

I think the ones with an extra set of stays between the rear dropouts and the seattube where teh top tube intersects the seattube might be stronger, or might be lighter for the strength. The definition of a mixte frame has been somewhat fluid over the years, but I consider a frame to be a mixte frame if it has this third set of stays.

Instead of using an extra set of stays, Thorn uses an extra tube in front of the seattube to add that reinforcement.
https://www.sjscycles.co.uk/thorn-st...rames/?geoc=US

To get on a bike, I bend my knee sharply and swing my knee over the top tube in front of my saddle. Have you tried that? You have to stand a bit further forward to do it that way and you might have to lean the bike more to the side. It helps to have a frame with a steeply sloped toptube to do that, but my rando bike has a horizontal top tube and I can get on the bike that way on that bike too.

When I was a kid I had a paper route (that really dates me, doesn't it?). And I had some giant steel baskets on the back of the bike and often piled up more bundles of papers on top of that rack. I learned as a kid that it was easier to swing my knee over the toptube than it was to swing my leg over that big pile of papers on back.

And, one last option would be a folder like a Bike Friday, a lot of people tour on Bike Fridays.

A neighbor several years ago put an old Bianchi Advantage mixte frame bike in the garbage. Lugged frame and fitted with a triple crank and six speed cluster, probably from the 80s. It was extremely low mileage and in very good shape. I brought it home and use that on my indoor trainer for exercise in the winter. If it was a taller frame (it is quite short and my seatpost is extended beyond the safe criteria) I would consider it for touring if I had a light load of only rear panniers.