Well that is a start. That many sides are going to lose heat.
Is the room something you are in all the time (going to heat 24-7) or is it a once in a while thing???
Do you have Natural gas available??? We have one of these heaters in the basement (playroom) It works excellent.
ProCom Blue Flame Vent-Free Wall Heater — 30,000 BTU Output, 1000 Sq. Ft. Heating Capacity, Model# MD300TBA | Dual-Fuel: Gas Propane Heaters| Northern Tool + Equipment
You can pick find them all over and do not need to be vented. The one we have is a little different it has a log in it to make it look like a fireplace. They also have them in radiant. Eh I like the regular ones better. The down side is when they run a lot they put moisture in the air. I guess that is also an upside in the winter. The moisture if ran a lot on high may condense on the windows. It is nothing as bad as the moisture from a shower. Got a friend using one to heat his entire house and works great but he was having moisture problems. (we were guessing that was where it was coming from) Something easy and kind of cheap. They have them from 10-30,000 BTUs. I would get the larger... It has a generic style thermostat.
I got mine cheap off of craigslist with a wooden surround.
We use the room regularly, but not all the time. A few times a week....to get away from the kids.
I have a heater similar to that (wall mounted, vent free) in my garage. They crank out the heat, but I don't really want that in the sun room. I really only have one wall not almost completely windows, and I'm saving that to mount the mini split. No heat in the winter means no A/C in the summer, so I'm really interested in the mini-split.
As cold as the floor gets, we are able to get the room to a decent temperature, but I don't want to have to run the heater on "high" all the time. so I'm thinking with better floor insulation, the mini-split and the quartz to supplement, we will be good to go as cold as it gets.