Pretty much the only solution is a stop message, then a program change message, then a start message. Octatrack is the same way, it plays through its pattern, then changes. This is configurable but it takes at least one pattern step, cannot make it immediate. I’ve gotten mine to switch instantly using this method.
I don’t know when the rytm sends its program change, but if you transform it into those three messages you should be able to get the digitakt to switch right when the program change is sent. If it sends it a few bars before you need the change, I can’t think of any way to fix that with midihub. Midihub doesn’t know how long your patterns are. Hopefully it is right when you want the switch.
I guess you need two transforms to make two additional messages out of one. I would try turning the program change into a stop ‘before’ and then the next transform a ‘start’ after.
There may be side effects to doing this, can’t remember. I mostly gave up on elektron sequencers. They are only superior to toys.