I have a pair of Cannondale Roam shoes that I got two seasons ago. They are great shoes, and I'll be wearing them a long time. They are great with double socks over 30 degrees. My toes get cold but it is bearable. Over 40 degrees is even better. This morning though it was 20 degrees on my 40 minute ride (I'm VERY slow right now, I just got back on the bike).

I tried a number of things today. I made insole insulation with wool and foil. I cut off the toe section of some old holey wool socks and wore them as leg warmers on my calves. I wore Louise Garnaue Stop Zone covers. I thought warm thoughts!

I got to work over 3 hours ago, and my toes are still very very cold. Tomorrow it'll be the big boots and platform pedals. Enough of the freezing!

When I get some cash, I'm going to try electric socks, but I don't know if that'll help the toes out or not.

I am not sure if your MTB shoes have vents in them but if they do find a way to seal them off. Toe covers full booties or just plain duct tape. If you can even slow the air a bit it will help.

one solution - expensive but it does mean you don't have to lose the clipless pedals.

https://www.sidiusa.com/diablogtx.htm

https://www.sidiusa.com/toaster.html

Another option is the lake MXZ302
https://www.lakecycling.com/category.aspx?categoryID=36

Yeah, the price is nuts. I got mine on e-bay over the summer for $80, new in box. They're excellent. I really can't say enough good things about them.

Have you tried fleece lined booties? I wear a pair of fleece lined booties over my Performance house-brand MTB shoes and I'm warm all the way down to 15 degrees for my 7.5 mile commute.

I love my cheap 661 Nuevo. They were $25 at price point. They now have over 2K miles on them and still fit well and work great because of the soft sole so I can walk in on the hard floors with out skating across them.. I ride them with SPDs but I also ride my junker with flat peddles with them. They are warm in this colder weather also.

+1 for fleece-lined shoe covers. I have PI AmFIBs, and they're pretty warm, even with my extremely well-vented mtb shoes. With those and some wool socks, I stay pretty toasty.

Different people definitely have different physiology, and it's really tough for some folks to keep toes warm. Even with the best equipment, my wife can't keep her feet warm without a heat source. Those battery-powered warmers sound great, but super-pricey. You should be able to find disposable toe warmers for about a buck per pair if you shop around.

I use the Diadora Patrol for my cheap winter cycling shoe. With some shoe covers they kept my feet warm through winter last year. Fairly easy to walk in too.

I don't think i've heard any decent suggestion of how one can keep their feet warm while attached to a thermal conductor to a large piece of metal exposed to freezing temperatures. It's like standing on a metal platform barefoot and trying to keep your feet warm by putting blankets over your feet.

SPD winter boots work well. No problems here.

I have some Shimano shoes which are great in 3 seasons. When it gets < 20*F though, I put on the platform pedals and wear boots. I'd like to get some Lake shoes but the platform pedals were cheaper.

I tried toughing it out with the MTB shoes, but finally at about -15*F last winter I started getting seriously concerned about frostbite and decided to do something different.

I've got some cheap hiking boots I wear most of the time, I think I paid $25 at KMart a couple of years ago. Nice and loose so I can do a few layers. When it's really cold I switch to high top, thinsulate lined hunting boots, but they're tiring because they're pretty heavy and my ankles don't flex (which I find VERY tiring) - but they ARE warm.

I've been OK down to 0°F by layering SmartWool socks, Mysterioso M-Tech Socks, and Seirus Hyperlite Stormsocks under my Shimano SH-MT20s, all wrapped in Sugoi Resistor Bike Booties.

Just get some full shoe covers.

I have some cheap covers from Performance and my feet are fine at 20F with only one pair of wool socks. My Diadora shoes have huge areas of mesh on them as well.

Nothing beats plain 'ol winter boots and platforms for warmth and cost.

I've got the fancy lakes, and they're OK, but once it gets below -5C the platforms go on.

+1 to PI AmFIB shoe covers + wool socks. The coldest that I remember riding with them was -15C/5F, and I don't remember having a point where my feet were the weak link in my gear.

The only thing about the shoe covers for commuting is that walking around in them will tear them up rather quick, so they're not an ideal solution if you also plan on, say, stopping at the grocery store on your way home.

I've got Roams but I've never used them below ~20°F. At that point I'm breaking out the Lakes or Answer boots.

You can probably squeeze some more mileage from you shoes with good socks, plastic bags and neoprene booties.

Make sure your not choking off circulation, your toes need some wiggle room to keep the blood flowing

I've got Answer Kashmir SPD boots that I use in the 20s (F) and below. You really need to keep an air gap between your toes and the front of the shoe. Jamming extra socks in there just ensures you get a conduction path to the front of the shoe.

I am hunting for a used mountain bike at the moment for a winter commuter. I have tried just about everything and nothing keeps my feet warmer than 1 hr below freezing.
I believe the MTB will have platforms or some type of touring pedal so that I can use regular boots.
Heck, this morning I was sitting on a heated bus wearing boots with wool socks and my toes were still cold.
Maybe my insanely warm hands are responsible for my cold feet.