Try using your 18-135 at specific focal lengths (24, 28, 35, 50, 90, 100 & 135) to see what you like then maybe try a couple of vintage lenses. Manual lenses are really satisfying and you get a lot for your money. A 50mm f2 will only cost you £15 whereas a 100mm f4 macro, scored 98/100 on PF will still only cost £125. I have the takumar version of the 100mm macro and it is astonishing. The other lens I would grab if I was limited to two is a Miranda 24mm f2.8 macro which renders beautifully and is a nice focal length. Mine cost £40. Manual lenses give you the opportunity to dabble with primes without breaking the bank
if you want AF, the smc da 35mm f2.4 is great for the money. A bit plasticky but works well for significantly less than £100. Also, the focal length works well