I haven't nailed down the exact criteria, but I've been testing it with 20+ minute long RAW LT files from my Canon R5 C. It hangs at the very end, apparently just before it writes the moov atom to the file.This happens with ProRes 4444 as the export ...
Yes, the dedicated hotshoe (Canon GP-E2) is discontinued, but you can still find them on the secondhand market. You'll also need the adapter (Canon AD-E1) but I can confirm that if you have both, it works with the R5 II.