I would do both. Foam board with spray foam in the cracks.
Curious did you consider framing it out and getting spray foam type insulation put in, that shits done awesome in my house
Looking for the update.
Good luck firing your welder. As for progress - sounds like you are going par for the course. All real weekend progress is accomplished in the last 2-4 hours of the usable weekend.
I sell IMP's (insulated metal panels). Ive seen a recent influx of trailer builders using them due to their low weight/high strength and low cost. Basically just .875" thick closed cell foam sandwiched between aluminum...but many different options are available regarding the cores and clads.