Definitely garage, and ideally an old bike you can leave on there.
I have an old fleece jacket i have for turbo, wear it for warm up, then off for main effort, and sometime back on to cool down., usually wearing just a thin base layer even when it is freezing outside
For me, turbo is for doing the hard, shorts efforts in the week, then leave the zone 2 work for longer riders outside at weekend, unless weather is really poor...
If you have a garage, it is probably the best option. Start cycling and you will start feeling warm in a few mins, it is like running in cold. the beginning is hard and then you just can't stop how good it is in cool weather. Also maybe do some research on the indoor cycling apps, which will suit you best, that can bring you even more motivation. I'm using Rouvy for the possibility to ride real-life routes and cannot complain about the winter trainings anymore.
I googled this for an air resistance non smart trainer.
Apparently the what 3 words location is: metal.recycling.bin 🤔
I used to have it in our garden cabin thing. Chilly to start but soon warmed up a bit. It used to be I stored my road bike in it for ease. Maybe if I had had internet down there then and not bought something that would drown out the noise of Concorde taking off at 60rpm it would still be there and not in the corner of the tool shed stopping me opening a cupboard.
Cold garage and a big fan.
I'm torn between the "large appliances" or "scrap metal" sections at our local tip.
Cold garage and a big fan.

I’ll just have the cold garage on its own, thanks.
Rear conservatory for me and turbo set up 7-8 months of the year so I can just hop on.
The conservatory pvc doors have pull out fly nets (previous Australian owners, seems standard there) so I have doors open, fly nets pulled across even when its been -5c outside. Cold to start but I very quickly warm up and the open doors limit the condensation.
Its also a nice bonus to turbo whilst looking across the garden. Fortunate that it's a rather lovely space so theres an inkling of being out amongst the trees.
