More bathroom photos

You may be able to insulate the walls. There is blow-in insulation or foam that is put in place through holes either cut in your drywall or in the exterior wall. Siding can sometimes be removed and replaced after the holes are filled. With brick, They have to go from the inside. Fire stops in the stud walls can mean LOTS of holes, though, I would think.

All of my outer rooms with two exterior walls get very cold, too. I have brick siding. I have lots of blown-in insulation in the attic of our 1978 ranch, but I don’t think there is much, if any in the walls. I freeze once the weather turns cold, lit is like feeling the warmth sucked out of my body. We solved this in three rooms in our house using something called cove heating. It is electric heat from panels hung up by the ceiling. He it is in our small bathroom. It works best hardwired into two household circuits to make 220 circuits (or is that 240? I never remember). It uses less electricity that way. You put a thermostat in each room with a cove heater, and keep the door closed except when entering and leaving. The room can get quite toasty. The cove heater puts out radiant heat. You can feel it “shining” on you when you stand still. The radiant heat hits objects in the room, heats them up, and they then radiate heat into the air until the room reaches your set temperature. We first got one for my father-in-law, who had the corner bedroom, and who was 90 years old. It worked very well and did not seem to add very much to our electric bill. Then, when we remodeled the bathroom, I had one installed there. Husband’s office/music room/library with French door got the third one. He uses it when he works from home or is practicing music. We tend to turn up the heat an hour or so before we want to use the room. The bathroom thermostat is set to turn on before shower time in the morning and go back down afterward. Unlike forced-air or fan heat, there is no breeze created that battles with how warm the extra heat versus the increased draft feels on the skin. There is nothing that things, kids, or pets can touch that might burn them. Mine come from Nebraska and are American made.

https://www.radiantsystemsinc.com/get-comfort-cove-pricing/specifications-sizing-guide/

We have a heated floor and a mini-split in our bathroom. Works well.

I recently replaced the flooring and shower, so radiant flooring is not an option

You might be able to have blown in insulation for the outside walls. However, the cove heating suggestion from Nancy sounds like something to explore.