If this is simply so you can monitor fx at zero latency, then the easiest and cheapest option is to simply half-normal/mult (assuming you have a rackmount patchbay) your preamp-outputs so that you can retain your inserts, but use the mults for aux / monitoring duties - these mults can be hardwired to a very small mixer that doesn't have to be great quality if it's only for monitoring.
I do this in my setup, except my mults (reverbs and delays for vocal tracking mainly) are simply patched back intop a spare input on my audio interface - i use the interfaces mixer to set monitoring levels on fx rather than a hardware mixer.
Works great.
If you are looking for a mixer to submix 3/4 tracks into a stereo recording then the above still holds, you just need a high quality 4-channel line mixer - which does seem to be a difficult proposition - the SSL X-Desk appears to be one of the smallest that ticks all the boxes.
The following will tick all the boxes in a single rack space, but it's not exactly high-end, plus it's more channels than X-Desk:-
Rolls Corporation - Real Sound - Products RM219 Stereo Line Mixer