Those were my two original lenses for my Pentax system. I moved in several directions seeking to improve my work and lessen the inconvenience, especially looking for economy-yet-sealed lenses for my K-7. Finally I had to sell off many things, mostly due to (non-photo) budget issues including 18-135 and the K-7. I tried a single step down, selling K-7 and buying a k200d, but the 18-135 really requires either K-7 or -5 to do its best, the in-cam corrections or PP is needed esp. over 100mm with my copy.
After a month or so I landed a used k-x, and started thinking of a minimum kit.
Last week I picked up a bargain da-l 55-300, so
I have both of them back again. I still have a very nice 18-200 Sigma, but plan to let it go soon. Not doubting the inconvenience of an occasional lens change, but also cannot doubt the IQ and general talent of the DA16-45 and 55-300 in either variant. I'm keeping the equally-talented 50-200wr (a rare early-day wr holdover) for when size trumps range and several MF primes for IQ when time to compose available.
Perhaps you could think of a 'tweener lens to overlap the gap? Do you shoot mostly in the 28-70 range with those lenses & that's why you swap as you do? Perhaps not, but it was worth a go from my side