Target Modes?? Where is target BOOT MODE

I have a rather important, for me, problem/question that I can't find an answer to, at least not a consistent answer, or one that doesn't involve a lot of tedious manual rebuilding and would still be missing quite a bit information, solution.

I have two 15" MB Pros, one a late model 2016 (A1707) the other the 15" 2018 MB Pro (A1909).

Both are running Mojave BUT slightly different versions. (I use my older one for my Alpha/Beta MacOS testing)

I recently damaged the screen on the 2018 MBP. Before I send it in for repair, I need to make a complete back up, a "clone" is my preference, a full TM backup would do .

My question is this...

I AM able to 'mount' the internal 4TB drive of my screenless/headless MBP on my earlier model MBP using "Target 'Disk' Mode".

I can access the drive BUT I am only able to assume it will also boot but can't be 100% certain.

I can & have 'copied' some of the 4 TB of content elsewhere, so I don't think it's corrupted. .

Much to my surprise, it seems that MacOS has lost the ability to use the older "Target BOOT Mode" (We used to be able to not just 'mount' a connected HD, we were able to 'boot' from the connected and selected HD, at startup.

(ie, my older MB Pro could select my newer screenless/headless MB Pro, as another HD to BOOT from. Allowing us to 'clone' or, worst case, get a usable FULL Time Machine backup of my screenless MBP's HD.

Before I send and spend to get my 'repaired' 2018 MBP back, if the HD gets trashed, (which seems to always be the case), I could, at least, do a FULL RESTORE.

I desperately want more than just files off my headless 2018 MB Pro.

(just the thought of having to reinstall all my software and manually recreate all my "personalization" settings and licences etc, has me ready to slash my wrists rather than have to go through all that. (jk)

Bottom line, did we actually lose the original capability of Target 'Boot' Mode to Target 'disk' and/or Target 'display' Mode ?

(before the obvious "you should have made backups, if the info is so valuable".

YES and I actually do and did. Unfortunately, for me, I was, literally a few thousand miles out to sea, sailing on board a 'professional yacht racing' sailboat (sailing 24x7 for 5,000 to 8,000 miles at a time) when we ran into some of the worst and most violent weather and wind conditions, in the southern ocean, that I have ever experienced in my 50+ years of yacht (sailboat) racing.

I've sailed in every ocean on the planet and have NEVER even been remotely scared, until a recent adventure. (35 to 40+ foot seas, winds over 70+ knots (~85-90 mph) on a 65-70ft stripped down "louge" on water, traveling so fast the top tier Mavic II drones we used to get off boat aerial videos were left in the dust before you could blink. They can travel over 45-50mph. The "flexible flier" sleds we're sailing hit speeds that had us all convinced we were going to barrel into 35ft seas and die trapped in a submarine with portholes everywhere.

Not too far off, one night about 3-4am a ginormous wave caught us from one side, swept over the boat, filled below deck with knee deep COLD water and ripped the mast and sails clean off the boat. (it's prudent to have harnesses on, and life lines shackled to anchor points all over the boat. Otherwise anyone "on deck" is GONE forever.

Those 'off watch' and below decks (usually sleeping) got greeted by CCCOLD knee high sea water and anything not lashed down became a floating projectile, people included, being tossed around like 'Mexican jumping beans'.

Sorry for the tangent, hard to forget that kind of experience.

Anyway my three 4TB Glyph 'Black Box' backup drives were KIA. My now, veteran, MBP was locked down in a sealed waterproof 'sea bag'

Is Target BOOT mode really gone? Any suggestions or known work arounds. I was not able to get Time Machine ruining on the older MBP to select the newer 'mounted' drive as the source to backup. That kinda surprised me but. Nor could I get CCC or Super Duper to clone that mounted drive, also a bit of a surprise. When did target booting go away, or did it and I just can't figure out how to?

TIA for any help, ideas etc.

Cheers,

Peter