Selling the OM-D in favor of a fully fledged DSLR?

You already have plenty of useful lens covergae in the wide angle with 12mm (=24mm FF). So no pressing need in that direction if money is so constrained.

I know I love the 9-18mm, but given financial restrictions, 12mm is already quite wide and I often shoot the 9-18 at 12mm or more.

All you would need is on the tele side. that can be accomplished with the OM-D with a 40-150mm or a 45-200mm for about $250. If you are truly into wildlife or birds a 43 70-300mm would work sufficinetly well, bought used to keep the price down, would very effective.

The OM-D can shoot very reasonably at iso 3200, thus the minimum aperture is not too much a concern, except as you point out for a stop or two of DOF. And that is not an enormous difference.

So I think you can reasonably enjoy the best camera that cam along in a long tine as it is, or just one tele lens for around $250.