OM 100mm f2.8 is wonderful little lens with a such a nice rendering

Miss mine. Wish my son would stay still so I can use such lenses.
Anyway 12mm is 18mm on FF. You could go full on samyang

18mm-35/45mm-75mm primes are all nice and small. If I am not mistaken bar the 45mm the other 3 also have the same filter size.
Personally I'd go for either 16-35mm f4 or tamron 17-28mm f2.8 at the wide end since you use that a lot and minimise your lenses changes and then buy a prime or two at 35mm-100mm ranges. There are plenty small f1.8 options in that range and all them are very good so you can't go wrong.