First, massive shout-out to Cloudschatze for not only modifying then sharing the updated BIOS for several Tandy models to enable booting a PicoMEM booting from CA00, but also for sending me a ROM with it burned already when my DIY project was delayed by...international relations. 😎 Thank you, sir!

So, now the TL/3 can boot to a 2GB DOS 6.22 image mounted from the MicroSD in the PicoMEM while leaving the D000 space available to map for EMS or UMBs...

With this CONFIG.SYS setup...

1FILES=30
2BUFFERS=10
3DOS=HIGH,UMB
4DEVICE=C:\DRIVERS\LOTECH\LTEMM.EXE /P:D000 /X /N
5DEVICE=C:\BIN\PICOMEM\USE!UMBS\USE!UMBS.SYS D800-DFFF
6DEVICE=C:\BIN\DOSMAX\DOSMAX.EXE /R- /Q- /P-
7DEVICEHIGH=C:\DRIVERS\REV2486\REVTO486.SYS
8SHELL=C:\COMMAND.COM /E:256 /P /H
9DEVICEHIGH=C:\DOS\ANSI.SYS
10LASTDRIVE=Z
11FCBS=4,0
12STACKS=9,256

I end up with this MEM /C output.

Hooray! Over 600K of available memory, 2MB of EMS, and 73K of UMBs used with plenty to spare sounds like I'm living the dream.
Except it's unstable as heck. Long story short, DOSMAX is enabling the use of both UMBs and EMS simultaneously, but the cost is it doesn't want to run many apps reliably. DOSMAX, however, is the only utility to successfully pull this off as The Last Byte so far just yields 64K of UMBs with the PicoMEM (same results with a Lo-Tech 2MB or 1MB board).

Anything else I should try utility or TSR sequence-wise? Any other settings I could futz with on the PicoMEM? It's been a fun & challenging cat 'n mouse game so far, but I think I've reached an impasse and could use a hint. I get similar results pulling the PicoMEM and running instead from a XT-IDE board and Lo-Tech 2MB (or 1MB) combo. It may just be this is the price to pay for running Tandy hardware with all its unique benefits, quirks, and limitations.