OK, I've tried this multiple times now without success.
I just bought a refurb from Amazon UK and it turns out it's a T515XXS8BUC2 (I was expecting a T510), from CHO (Chile)! So I have successfully (though stressfully) managed to restore from various levels of bricked using Samfw.com_SM-T515_CHO_T515XXS8BUC2_fac.
I have successfully unlocked the bootloader (can confirm it is greyed out when in stock ROM, as already unlocked, and also shows as unlocked when entering download mode).
I have tried flashing both 'twrp-3.7.0_11-0-T510XXU5CWA1-20230330.tar.md5' and 'twrp-3.6.0_11-2-T510XXU5CUL1-20220218.tar.md5' to the AP, using Odin 3.14.1, with 'Auto Reboot' off. On success, I press Power + Volume down to 'cancel' and immediately press Power + Volume Up to enter the TWRP. However, after the boot logo, it immediately goes to the warning screen stating 'this tablet's bootloader is unlocked and its software integrity cannot be verified", from which it enters a boot loop and I can't get any further.
Thanks to the suggestion of waiting until the screen is off in the loop cycle, before pressing both volume buttons and jamming the USB in, I've been able to recover from here to reinstall the stock ROM.
I would love to upgrade to Android 11/12, and to be able to install a newer WebView for my kiosk use case.
Any tips on how to get TWRP actually installed on this device would be greatly appreciated as I'm stuck.