OP Update! We had nine magical days in England (with a quick stop in southern Wales). I was right that adding northern Wales would be too much, and I'm glad I talked myself out of it.
Cheltenham/Cotswolds: We went to the ruins of Hailes Abbey, but it was already closed for the season. However, we were able to observe over the fence from fairly close, and it was interesting. There's also a tiny 11th century church immediately next door that was open - that ended up being a highlight of our day.
Avebury/Silbury Hill/W Kennet Long Barrow: We did this as a day trip from Cheltenham. Totally do-able, even including an extended walk around all of the Avebury stones and a cream tea at the Trust cafe. We walked up to the long barrow and, if you go to Avebury, DO NOT miss this! It was a beautiful and easy stroll up the hill with gorgeous views and a chance to go inside a portion of a Stone Age burial chamber. Absolute trip highlight.
Tintern Abbey: Absolutely breathtaking. We spent 3 hours or so exploring the ruins, including the excellent plumbing still visible. This was the 8yo's favorite (and probably mine too). We ate tea at the little tearoom right next door, but then realized there was a whole little village about half a mile beyond that probably had better options.
Ludlow Castle: Fantastic. We tried to do this after Tintern, but we spent too much time there and then the castle was closed by the time we got to Ludlow. We unanimously decided it looked cool enough to go back to on our way out of Shrewsbury though, and we weren't disappointed. So many nooks and crooks and excellent "poop chambers" (as my 8yo called them - garderobe to others). Definitely buy the 1pound guide on your way in - we bought it on the way out and wished we had had it earlier.
Shrewsbury Castle: This was closed on the day we were in town! I should have read ahead. And you can only see small portions of the castle from outside it; it was not terribly interesting, sadly. Next time! :)
We did also do the Tower of London after all of this. It was super - but for us, hardly anything could compare with being able to walk around and in and touch the stones of Ludlow and Tintern Abbey. In short, a fabulous trip.
Thanks for all the excellent advice! It' was so appreciated.