I was in the same spot you were in.
Was real close to pulling the trigger on the A4 first, and then found a good deal on a Rytm at the last minute, and decided to get it first. The A4 followed a month later.
Now that the dust has cleared, I’m glad I went in that order. Of course, I didn’t have a MD UW like you, so I might have gone A4 first.
But from a learning perspective, the A4 made a lot of sense thanks to my time spent with the Rytm. They’re really made for one another, and there’s a good chance you’ll end up with both.
A4’s drums are remarkably good, but you really gotta dedicate two tracks to drums if you’re trying to do it all on one Elektron machine. The kicks really need full use of the envelopes, unless you like short little electro- knuckly knocker kicks.
I’d watch some Rytm and A4 tutorial vids (cuckoos are great), and see which one gets you more excited and go with that.
I think you’ll still find room for the MD UW. Elektron is clever in their overlap of products. Each have definitive strengths. A4/Rytm/MonoMachine is my chosen trinity and I feel each machine to be quite different, yet complimentary. Plus, MD gives you an external midi sequencer if you want to add anything else down the road.