Even with IBIS, I'll bet the 800mm F11 will do better with some strategic bracing, rather than handheld. I chase my butterflies at home with lenses from 600-840mm equivalents and find them pretty mobile until I rest something on a stable platform, even if its just a thick pillow on a window sill. B&H says my R5 won't show until late October now.
Canon M6 Mk II w EOS 400 DO shot from a bedroom window
Olympus M1X with Oly 300 f4 + 1.4 TC -> 840mm equivalent at ISO 1600