We have a 5x4m roughly 33mm thick log cabin that was delivered as a kit and built on site by the supplier on an existing concrete base.
I know you’re looking at self build but it was built and usable in 2 days (month lead time). It would have taken me at least a month of weekends to do something similar during which time I’d have done stuff all with the kids and not been riding.
We were lucky in that we could afford to pay for someone else to do.
The construction is pretty warm if in there for a couple of hours (I just have a little 2kw convector on in there on low when temperature gets below about 10deg). It’s a relatively small space so it doesn’t need much. Some insulation in the ceiling might help too.
Disclaimer. Mine mostly used for bike fettling / woodwork not sit down crafts.