If anyone comes looking for the same issues I described in my post above about my 2500 XL/2... I solved it (but now I’m stuck with my 1000 TL/2).
The 2500 XL/2 needed:
This small ISA ROM card:
https://monotech.fwscart.com/DoubleROM_-_IDE_ … 4_19995208.aspx
This ROM card sidesteps the Tandy 2500 XL/2's BIOS' IDE controller-related functions. I had to go into the Tandy BIOS setup (setupxl2.exe) and mark both primary and secondary drives as "none", since the ROM card fills in for that stuff.
And a Sound Blaster 16 CT2290, which sidesteps the Tandy 2500 XL/2's crappy integrated IDE controller. It wasn't enough to have the ROM card, since the Tandy 2500 XL/2 IDE controller cannot handle the compact flash cards I've tried to use. I set the SB16's IDE controller address to 170h, ran the ROM card's XTIDECFG.COM config tool, selected "detect..." and everything came up just fine. It even ignored the Tandy's internal IDE controller, telling me there was only 1 IDE controller (the SB16's).
Rebooted and the card works fine as a boot device, even though it was partitioned and formatted on a much newer PC. YAY!!
EDIT: PS: I eventually got the 1000 TL/2 working with an XTIDE BIOS card that also has a CF card socket on it... but the Sound Blaster clone (seems to be an ESS Audio Magician Plus) freezes the machine on using its DAC, despite having set it to IRQ5 and DMA3 (the Tandy DAC is bound to IRQ7 and DMA 1). The Tandy DAC works, though.