I doubt you'll do better, in terms of IQ, than the Tamron you have. If that worked for you, look for a prime or 2.
I bought a crop body recently to add to my FF, thinking it would add more variety to my lenses. Actually it looks like it will do the exact opposite, ending up with lots of overlaps, or the same, depending which lens gets put on which body. No point putting the 135 on the FF and 85 on the crop, for example. Also means I will actually have to think when shooting (which lens on which body do I switch). I don't get back into serious shooting for a couple of months but I think I'll be replacing the crop with another FF, sensor confusion being just one reason.