I live in Wisconsin. I know that to install a hyper heating mini-split system in my 20' x 24' garage would cost about $4000, including install. The garage is a hobby workshop. Would a DIY mini-split work IF I would use a couple of portable oil heaters when the temperature gets too cold for the mini-split to work? I used 2 portable oil heaters in the garage this past winter and they heated the garage fine, just expensive. I'd prefer not to spend $4000 on a solution when I won't be in the garage but a couple of days a week. Thanks for your advice.

Sounds like lack of insulation is your problem or very bad doors but yes a Mr. Cool minisplit that you put in should work. There are tons of videos on installing them. You are going the right route.