I don't know if this will help. Fludrocortisone did not work for me the two times I tried it. I just got bad side effects.
I do take diy electrolyte mix but that didn't help the numbers either. Next I added licorice. (Be careful with this and research the warnings.) It moved my average of 84/58 up to 92/59--not much though.
What worked best is when I started Mutaflor probiotic. I started to notice that within a short time after a dose, I would start to get shivery and sweat a bit. Then I remembered that whenever my mom's bp goes high she gets the same way so I took my bp measurement and it was something like 104/68 and about an hour later 115/68.
I've never had anything do that for me. It also seems that after I had been taking it for a while it maintained that at least somewhat. If I'm having a really crash day it might go back down but for the most part the top reading stays over 100 now.
Not sure if this will help you because everyone's different. This ME/CFS patient has a website and he talks about Mutaflor as well as some other things that have helped him. His website is quite extensive. Hope you can find something to help.
https://cfsremission.com (He also talks about licorice.)