Never had nor dualbooted a Mac, this looks like a good resource on the topic though ...
https://www.lifewire.com/dual-boot-linu ... os-4125733Agree with much of what the guy says too, was going to suggest rEFInd as the bootloader, the guy Rod Smith, developer of rEFInd clearly knows his stuff and gnu/Linux + Mac is part of his stuff. I've played with rEFInd on a uefi-gpt system, it's certainly not bad regardless. I'd look over any documentation he has on the subject at his website for rEFInd. I also agree with them advising folks to try Ubuntu (or my preference in such Linux Mint)whatever is based on their latest LTS = long term support offerings. When uefi(+ secureboot) + gpt hit, I had a problem child system and wasn't yet up to speed, many distro's such as Debian wouldn't install normally. So as a shortcut I used Linux Mint's Xfce release on a 10-15gb partition, was just intended to smooth things over and could be used to boot non-uefi etc capable distro's.
Anyway, Ubuntu ( or LM's Ubuntu based main releases) are geared to widest range hardware possible out-of-box, surely this includes Mac/Apple hardware. Plus that tute suggests trying it out in live session prior to installing and yeppers that ext4 is supported. REFInd is perfectly capable of ext4 and with it handling the boot, doesn't matter if Mac decides to list them as "Unknown" or whatever when you're booted into it. Can't hurt to try Ubuntu, tute says 25gb's is the bare minimum Canonical Inc recommends for an install. Personally I favor all of 29-30gbs for Debian. Though I also do minimal netinstalls a Stretch install takes up all of 4-5gbs, that's after several years and I do keep most my shared data on a separate partition, a Buster install about the same anyway. Not sure what a stock Debian install uses in terms of disk space, they'll have those posted somewhere surely. Only ever once installed a full Debian OS and it was by accident long ago while attempting to do a bare minimal netinstall. Bare minimal netinstall's don't require super-nix-ninja powers or anything but are much more involved than a full Debian OS install in out-of-box condition. Would tackle one project at a time and installing something gnu/Linux to your Mac is obviously priority/proj #1 here.
One thing of possible interest, gnu/Linux support swapfiles as it has for a long time. People don't necessarily have to use a dedicated partition, I do because it's what I'm used to and it's the conventional method. Though people are doing much with swapfiles as well. Evidently even able to hibernate with them and whatever else. A swapfile is still going to take up X-amount of disk space and I also tend to have 2, sometimes 3 other gnu/Nix installs on the system, so also prefer having a dedi swap partition for that reason, they can all share it no problem. Of course being gnu/Linux someone can have a combo, bunch of swap partitions and/or files and assign priorities for how they should get used.
Sighs ... kind of jumping all over the place here and not done either.

Not sure as to exactly how partitioning works on Mac's either way. Whether there's some limit to how many, some other oddness etc. So mostly just going to shut it, though if the above is relevant, could always shoe-horn Debian in there(or others), perhaps use a Kvm virtual machine or etc too. Debian is very much capable of anything Ubuntu is(or buntu based)last I bothered checking Ubuntu still is vast majority Debian, just no longer binary compatible for their own reasons. Would still favor buntu ( or Mint) over Debian in this situation just cause of hardware support, less fiddling required and likely to be involved vs going with Debian in all it's glory. That's one of Debian's founding principles and obvious traditions, no proprietary out-of-box, that's certainly going to apply to Apple(Mac OS.) Just saying Ubuntu( Linux Mint) probably path of lesser resistance. At least until you get more familiar and comfortable.
Also have installed more than a few gnu/Linux Os's with only wireless connection as an option. Another reason for advising buntu/Mint, more likely to have any required wifi driver for you already. Will quickly become apparent during live session testing whether or not a wired connection might be involved. If you have an external usb drive, then don't necessarily need a thumb drive. Really is just a bigger thumb drive, errrr, sort of. Never had to use an external usb drive for the purpose can't see much difference, still just a block device, still capable of being partitioned (a size adequate to hold the iso and would go with fat32 as usual.) Using an external drive vs a thumb drive should be no problem, same, same really.
It's easy enough to get a bay adaptor for not all that much on a pc to turn an optical drive bay into a place for a second drive (like an SSD.) Not sure if that's the same for Mac, not seeing much reason it shouldn't be. Something to take up later, when this plague scare calms down. It's causing all manner of weirdness here in the USA too, in my state the Governor closed all public offices to the public until Apr 6th. I'm not getting what all the hubbub is about it. An avg flu season supposedly kills 22thou people per yr here, mostly old and really young. Mortality rate of under 4%, much higher than influenza, not omg we're all going to die by any stretch though. Haven't really been paying all that close attention to this but poking here and there. Data I looked over said 80% of the fatalities attributed are people 65yrs or older. Errrrr ... with you being an 68yr old retired programmer that's probably not as comforting as intended.
Also it's clearly not funny when people of whatever age are dying from a virus. Only saying yeah, mucho nastier than flu but people are acting like it's a prospective extinction event and the stuff I'm seeing on it clearly indicates that's ridiculous. Keeps bugging me, making me wonder what in the hades I'm missing here. Guess I should actually start reading about it perhaps. Of course I'm not going to slow dance with a bunch of people known to be infected with this bug, only not losing much sleep over it either. As a relatively healthy middle aged person. Would be taking it more seriously with young ones or esp elderly people I care about around. What's it like there, any credible stats you know of on this situation ?
Ending on a good note, your experience with Unix/Mac and the command-line is bound to come in handy with anything tech. Tackling gnu/Linux + Mac definitely shouldn't be overly tough.