I would get a 20F bag with full baffling like the WM Ultralite (I do own it and love it).
This is my reasoning:
With a a 45synth bag you are covered for wet summers. When the temp cools down a bit you can supplement with puffy clothing and probably get down to freezing easily (and the puffy clothing is nice in those chilly morning/nights). This I think is near your rationale.
Going from above freezing to below is a huge difference in perceived temp, because now you're body is moving from fighting hypothermia to fighting frostbite. So getting a 30F bag only lets you "play" around this significant temperature. You usual light insulation clothing for near freezing temps will let you push your bag colder, but you won't get the same 10-15F boost as you do when temps are all above freezing mostly because quality light 30F bags have sewn through baffles (as in the 45F down to freezing).
This is where I advocate a 20F bag. It gets you well into freezing temps. When the forecast calls for freezing nights, a 20F bag is warm enough to sleep comfortable should those all too frequent cold snaps occur, but not so warm that you can't sleep in 40F temps too. Then if you need, you can push a 20F bag down to 15F or 10F with light puffies and even further if you go with heavier weight insulation as forecasts dictates.
If you can take a synth 20 bag down to -10F with your MHW 45F then a proper 20F down bag will take you to same temps if not colder, with a considerable weight savings.
So with these two bags you will have solid coverage between -10F and 55F or so (based on your experiences) which is impressive. There really aren't too many holes in that range where you'll be uncomfortable, especially if you supplement with your clothing. It will make your EMS bag redundant but sell it off to friend or keep as a loaner.
Now for a specific bag, the WM UL has a trick, it's actually EN rated to ~16F and thus a very warm 20F bag for a full pound savings on your EMS. It is pricey but I've not regretted my purchase once (ok, maybe summer in the desert...but a MYOG quilt is in the works). I've slept as warm as 50F in it and it vents well enough.
For reference this is my system; unfortunately I don't get many chances to test "really" cold temps or soggy conditions in the desert so take with a grain of salt and I am conservative since I hate the cold (although slowly adapting). Pretty much the WM UL for all temps I routinely see. In the heat of summer I've got the synth quilt in the works which will also be used in wet conditions. If the forecast calls for 20F but I'm at elevation, the synth quilt comes along as extra protection for cold snaps. With appropriate clothing for the season I'm fairly confident that I can get down to 0F using these two (my synth quilt is lighter than a MHW 45F). I'm sure playing with VBL would go further. I did just pick up a 0F bag to test this upcoming winter but I expect it will be relegated to deep WY winter conditions or high altitudes since the rest of my system is so versatile.