Advice Wanted Which combination for photographing small things (birds, wildlife) at long distances

I have two camera bodies: OM-1 and Fuji X-T5.
I want to upgrade a long lens to either the Olympus 300mm f/4 or Fujinon XF 150-600 f/5.6-8 lens.

I am mostly interested in anyone who has experience with these two lenses and their evaluation of image quality. (I am not that interested in BIF so the differences in the two cameras is less important there.)
I know the Olympus costs about US $1,000 more
I know the Fuji is a bit heavier and longer

Thanks for any insight.
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I should have mentioned that my existing long lens is the Panasonic Leica 100-400mm, which is excellent lens and much lighter than either of the potential upgrades. I just seek a bit more quality and am looking for comparative experience with the Olympus and Fuji alternatives.
looking for comparative experience with the Olympus and Fuji alternatives.
My experience with Fuji's 100–400 was pretty bad. Soft, spectacularly so with the dedicated TC, the detachable foot's vibration prone due to poor design, and the OIS frequently turns itself off. You have to turn the body off, mount another lens, power cycle the body, and put the 100–400 back on to get OIS working again. Was planning warranty the one I had as there was some downtime for it coming up but it was stolen as part of a in a break in first. Which worked for me too; kind of a relief to get rid of it. I switched to the Panasonic 100–300 II, which is better all around even though my copy has the usual softness at 275–300. Can't comment on the PL or Olympus 100–400s as I haven't tried those—thought about them but I'd rather accept a bit less reach for smaller size and lighter weight of a 100–300.

People rave about how great and sharp the XF 100–400 is, though, so I dunno.