I know exactly what you mean about dithering ideas. I know and you know that adding more is not the best bet, take the Brichardi that you returned to the shop, the brichardi I have returned , the Dickfeldi I gave away only to be returned to me, the Caudopunctatus I gave away

The 6 Multies I passed on (wish I still had them

)
I know an ideal set up that should work in the 4 footer but I'm still going against it by going for something that I actually want rather rhan what is practical
I think the Lucipinnis would work in your tank with what you have, it's still "busy" already, would give the Frontasa a miss, I would love them in my own tank, the Bathyphilus would need lots of bottom room , a big open space, means pushing all your rocks to either end of the tank, don't know if your current residents would be happy
It's true about the finacial side, because the Tangs are not a popular they are more expensive, it's a shame because although not as colourful they certainly are more enjoyable to watch behaviour wise
An example would be .. at the moment the Sumbu look nice in the shaded tank with more room and the 2 Multies are having a ball exploring all the holes in the ocean rock, the sumbus are going purple and the yellow on the fins looks lovely, I could add a group of Paracyps and a trio of Xenos and the tank would be made, reality is I will striop it down add much brighter lighting and different stock at much more expense, where's thet logic