I have an i7 NUC which I used for with GP for awhile but maxxed out the memory and CPU with the number of plugins I wanted, so was considering a new rig centered around a new NUC, just like you. The problem is the latest and greatest chips are still hard to come by/not available, and I did like and checked out the specs of the Xeon based NUCs. After further research it turned out that Xeons are not necessarily the best choice for audio processing. Between the cost and fact a Xeon based rig wasn’t going to be an audio slayer, and the fact that the latest NUCs are still not available (just checked simplynuc.com) I built my own in an 11 liter mini-ITX case by Streacom. The link to what I built is HERE. Just like as mentioned by others, I pack it around in a Pelican case with it’s associated gear, and it is everything I had hoped for. I’ve never seen it beyond 25% loading, it’s absolutely silent and it stays cool. But - it was expensive. By the time the dust settled the build and all the peripherals were over $4k. I personally don’t mind the 11 liter case, but it is substantially bigger even though it’s all in a case (screen, KB etc). I do a lot more with my rig than just GP and audio, so my rig is probably over the top for most other user’s needs.
Anyway, just my thoughts and recent experiences with this. I have hauled my old NUC around in my backpack all over the world without any problems, but it was more challenging, unlike a laptop just because of all the other unattached bits and pieces that you have to have. My new rig would probably fit into a taxi with the other gear you describe, but no doubt about it that it takes up a lot more space than just a NUC or laptop. I know nothing about Macs (although I love my iPad2!).
Normally I’d say go for the latest version (11th generation) of an i9 just below the “enthusiast” level - e.g. the CPU will cost you $500 - $600 instead of over $1000. The problem is that due to the worldwide chip shortages they just aren’t available in any form factor. Whatever you finally decide on, I highly recommend the fastest and as much RAM as you can afford with the largest, fastest M.2 NVMe SSDs you can afford assuming you are able to get a decent processor(s). At least it won’t disappoint and you probably won’t outgrow it. Good luck in your quest.