I think they’ll fit, but put the camera into crop mode (like Nikon).
That’s certainly what happens if you fit an ef-s lens onto an R5 or R6.
But I don’t see how that helps to do FF on a budget as instantly it’s no longer FF. however using 30 year old EF lenses is definitely a great budget option.
A £400 EF70-200 2.8L is a bargain, and the R6 makes it a better lens than the mk 1 IS. There’s loads of other examples of classic top tier EF lenses, or even great consumer lenses like the 28-105 which is optically great, built like a tank and is about £100.
Starting from scratch w brand new lenses, the new R series aren’t a cheap option, but for people who either already own EF lenses, or are prepared to think creatively then it doesn’t have to cost the earth.