Some exploring turns up the Canon 400mm f/5.6L lens. This is an EF mount lens that I would be putting on my Canon R5 using the RF to EF adapter that I have. This lens is available in good condition used for under $1000. It has a reputation for being very sharp.
It can be used with the 1.4 converter, but you lose autofocus. There are hacks to keep autofocus that involve putting tape over 3 of the contacts, but I hope to avoid that.
Another option is to turn on the 1.6 crop setting in the camera itself. This gives you effectively a 640mm lens, and you retain autofocus.
Bear in mind also that the R5 give you IBIS (in body image stabilization), which will be a nice help with this lens. The claim is that 3-5 stops of stabilization will be had.
Tom's Bird Info / tom@mmto.org