Not to be confused with a tenor trombone with an F attachment, I'm looking for a valved trombone in the key of F. I've seen the Schiller one, through jimlaabs, but I was wondering if anyone had one that they've already played, or a review of the alto valve trombones Schiller sells.

You mean high F, not low F, correct? The Schiller is really the only one sadly, and the few reviews I've seen have not been good. I've always been curious about them (as I too would love a high F valve trombone), but apparently they're a good way to set money on fire.

Weirdly enough, the better solution is to buy a Getzen Frumpet (a completely useless instrument stock) and swap the bell and leadpipe for small bore trombone parts. The result is a functional alto valve trombone in F or Eb. I plan to do this to my own frumpet eventually.

This was an incredibly helpful answer! It's cheaper than ordering an ISO or even designing a whole new instrument from scratch, which were seeming to be my only possible options! Thank you so much