Would this be a good tree to try and graft some shimpaku on? I’ve never grafted anything before
Yes.
Grafting is a major hurdle mentally, because it looks like it's a pro-move but it's not that hard if you take 5-15 minutes per graft and if you have the right tools. And absolute must is grafting tape or parafilm and a super sharp knife, as well as plenty of scions. Make sure you have 2-3x more scions available than you want to graft, and graft at least 2 times as much as you want to keep.
Scion grafting is the easiest, wedge grafting is more difficult because it requires way more alignment and technique. I'm practicing it on pines, but I can't tell if it worked until next spring.. Though the results from wedges are usually faster due to the fact that there are entire branches grafted on instead of lil branchlets.
Some people do not enforce the flap on the receiving tree, I found out this year that it's pretty important to do so. I'm using zip ties next year instead of wire, because zip ties are easier to secure without a lot of movement. My wire tends to be too rigid and pushed the scion around when applying it.
Since all junipers have similar structures, you can practice whenever you'd like on whatever you'd like. A couple of cheap pfizers or procumbens can get you pretty far. It's always good to have a sense of how such a wound heals, even if you don't insert a scion. The callus tissue will show you how scion alignment will work on different branch thicknesses.