I’m sure you are historically correct but to reiterate, & for the record, I was able to set up SPF on three domains hosted by Ionos just yesterday (20/01/2023).
P.S. The SPF trouble with Gmail seems to stem from changes brought in by Google since November 2022.
to which I alluded earlier 
You have a time machine? cool!
You did indeed 
Sssh! Don’t tell anyone (post now edited).
I know your problem. Gmail keeps bringing out new protocols so that you will get so fed up that you will leave your provider and go to Gmail. Crafty. If you enter this in a Google Search “how do i set up spf on ionos email” you will get some guidance notes. However, some have commented that it doesn’t actually work.
Good luck.
I found guidance contained within the Ionos site &, AFAIAA, the fix has worked.