Well, I've come to the limit of my endurance trying to convert a GPT+UEFI booting Linux install into a legacy boot (which is seems to me is the only way I can get the instruction for the boot process to check the optical drive first, to stick). I even updated the mobo Flash (clutching at straws, even though the website says not to update unless you have specific problems... my firmware was dated something like 2017 and there were a string of tweaks listed since then). The only success I had was by setting the mobo to legacy* boot only, and creating a fresh install on the HDD (with no SDD attached), unless you count discovering multiple ways to destroy a Linux system as "successes".
* Legacy is what I was referring to as MBR above - no, I wasn't confused: legacy boot needs a Master Boot Record to boot from, and GPT supports that too. It just happens that the (non-GPT) partition table is also held in the MBR... but it's called a BOOT record!
For expediency, I have settled for UEFI and its authoritarian ways... I shouldn't need to overrule it very often, and when I do I will just have to hit F2 during boot.
I bit the bullet and ordered a Chinese 4K HDMI KVM switch so I can have keyboard, mouse, and monitor on my desk, and the Zen and my notebook under it. In case that takes a while to come, Remmina works fine with the Tight VNC server already running on the notebook (SSVNC, as a Linux Mint VNC client, didn't).
Now to have a play with RAID. Overclockers have blown away my order for four sets of anti-vibration HDD screws, so I will engineer something myself using standard rubber grommets or even tap washers, but I have received the cables (0.9m SATA) and power adapters (Molex to twin SATA) I need. Four HDDs, SSD, and M Disc optical drive use up all six mobo SATA ports between them; if I want any other drives they will have to be adapted to USB3 (somewhere I have another DVD writer - but I expect that has IDE, so it is ideal to put on a USB port because the mobo only has SATA).
I have gone off the idea of RAID 1+0, because striping a pair of disks might well increase their overall read/write performance, but a striped disk needs its partner and a compatible RAID controller before it is readable. I have mentioned before that (years ago, on my AMD 486 home-brew with IDE RAID built in) a mains power glitch killed both drives of the mirror, and I recovered that only by patching in a motor control chip which got one drive running just long enough to pull the data off it. One drive of a mirrored pair is readable without special measures; one drive of a striped pair isn't, nor are both drives without the RAID controller to reconstruct the data.
By going RAID 1 (mirroring), I can have 4.5TB of storage (2 2TB and 2 1.5TB drives) instead of 3TB, insured against the failure of any one drive (or two, in the right combination), and if the mobo went tits-up I could still recover the data* by ordinary processes. Mirroring could still increase the read performance, if not the write performance. It's not insured against a surge on the 12V rail killing all the disks at once, but lightning doesn't strike twice, does it? Especially not if I put a power-conditioning UPS in front of it (which I have knocking about somewhere, when I unearth it and if it still works... might need a new battery).
* Or at least, I think I could - one objective of this "play with RAID" is to see what the raw disk looks like. Instructions always say about installing Windows drivers for RAID, but I never found that necessary - the 486 mobo handled the mirroring and just presented Windows with something that looked like an ordinary disk.
Incidentally, the PSU for my 486 system had a feed-through power socket for a monitor. Modern PSUs don't seem to do this, but I have a master+slave(2) mains adapter which works very well, turning the monitor on and off when the power demand on the master socket (feeding the PC PSU) goes above or below a threshold. I was worried that the monitor would still need user intervention to power it up after mains was restored, but it starts up of its own accord so that's great. Dunno how I'm going to tie that in with the notebook on a KVM...