I find 35 much more useful than my 50, particularly as a single lens 'kit'.
35 limited is wonderful, but it's not really a low light lens - more of a lens that can make good light look even better, if that makes sense? It's even better optically than the 15 limited, which you have, but I still prefer the 15mm angle of view. It's also more of a very close up lens, than a macro, unless you don't mind your subject touching the front element.
See my lens samples here, (photos 1, 2, 4 and 7 are with the 35 limited), so you can see it's very similar rendering and colour to the 15 (photos 3 and 6).
Does decent starbursts too, which can be pretty impressive if you can work them into a close up. Also it's 49mm, as you seem to want to keep your filters the same… I can understand that. If you need a fast focusing indoor lens, then you already have your 21mm.
https://www.pentaxforums.com/forums/members/13191-calsan/albums/7410-lens-samples/
hope this helps.