I thought about conversion kits but from what I understand, they are not much cheaper than a complete gun. I would also get the full size and see how you like it for range/home before I looked into a smaller one for EDC even though you were looking to do both with one gun.

Well since you already use a compact I voted buy a compact 320rx, I personally think the fullsize is to big for edc and I don't like sending any of my weapons that I currently use for edc away for work and then wait for its return. You know how well your 320 runs so why chance anything happening to it during the work being done. Also you would then have a back up fcu.

Mill the slide and get an RMR

I've seen too many issues with the Romeo 1's (cracked glass mainly) to suggest it for an EDC. Sure Sig will fix/replace, but that does you know good when the RDS breaks right when you needed it.

Trijicon RMR is the only carry grade MRDS from everything I've ready. You can get a gen 1 from Cabelas for about $350, if you want to put a little extra effort in, you can get $100 Cabela's gift cards for $80 on eBay, knocking the price down even more.

The big benefit of a full-size is increased sight radius which is irrelevant with the RDS. So that says to me stay compact.

The conversion kits seem to be going for most of the price of a full gun, so if you go Romeo, I'd buy a new RX vs a conversion kit.

If you don't feel strongly about the RMR, you could get a Vortex which is still much more proven than the Romeo and have the slide killed for $350 ish.