Well he died in 1574, buuut I conveniently did not mention his death (mostly due to trying to focus on English events despite there not being much of note going on at the moment). However the current time would be during his decline following the massacre in which he's not in sound mental or physical health. Doesn't rule him out, but does put said union into question.
In the original draft of the TL I planned for her first marriage to be to the King of Portugal, but of course that union was going to be a dud and with no heirs Portugal meets the same fate post-Sebastian as IOTL. In this draft it's a bit more realistic in that said betrothal was proposed but stalled/rejected thus never even happening. So Sebastian of Portugal is out (and will likely be dead in a few more years anyway, leaving daddy to take Portugal). Rudolph would be great if he'd stop being such a lothario (which he won't). Charles IX would be good if his death was butterflied and if he wasn't declining mentally/emotionally. Someone suggested Ernst, Rudolph's brother, not sure if he's married or not by 1575 though. If not he is an option/alternative. Could also turn to a English nobleman, though for a Habsburg Princess that would be unorthodox. Though she's also an English princess. James of Scotland is there too, though the proposal was declined by Phillip II of England originally due to Catherine being first in line and Scotland, well, being a threat. Perhaps now that he has a son though it could happen.
The elephant in the room would be the last resort "there is literally nobody else viable" option of Enrico Emmanuele, Duke of Savoy....and grandson of Anne Boleyn (and thus half-1st cousin). i.e. it'd be good that both Catherine of Aragon & Mary I are dead because ooh boy would they have a reaction to the mere idea of that.