Nice to hear from you again. I hope you're having a great vacation. I certainly approve of your vacation reading material .
When I started making dry plate negatives, I just carried over the basic processing techniques I'd used for years with commercial film negatives -- most recently Heico fix with half the recommended hardening additive. Works fine, but if you are interested in pursuing as historical a path as possible, you might follow the advice of T. Thorne Baker in Successful Negative Making, 1905. Baker wrote it as a short manual for using the newly available commercial gelatin dry plates. He writes,
"The quantity of hypo used in the fixing bath varies according to the make of the plate used, but in general a solution of:
Hypo ..... 5 oz
Water .. 20 oz
will be found to answer well. Some makes of plates contain a larger percentage of silver iodide than others, and these require a very strong bath, such as eight ounces of hypo to the pint of water." p33.
And on p35, "In hot weather it is sometimes necessary to employ a hardening bath, in order to prevent frilling at the edges of the film. Alum, chrome alum, or formalin are the recognized agents for hardening, and they are of power increasing in the order named. Thus a five percent solution of alum may be used, a one per cent solution of chrome alum, or a very weak solution of formalin. Blistering is most frequently caused by transferring a plate from a warmish developer to a freshly-made and therefore very cold fixing bath; it is essential to avoid sudden temperature changes when dealing with gelatin."
I don't know if you have any trouble getting chrome alum, but plain (potassium) alum should be readily available as a home canning supply. It's no longer recommended as such because of health concerns, but a lot of home preservers still use it and it's very inexpensive -- at least in my area.
Looking forward to seeing more gorgeous plates!