Of course, the 300 mm lens with its mass will help absorb and naturally quench the vibrations from handheld shots. In fact, with the huge 50MP of the Pentax MF sensor, even small birds in the tree can be cropped to a decent size.
But still, for a lot of birding, handheld, a lighter camera, (Pentax K1, Sony, Canon, Nikon, and also the APS-C or MFT image-stabilized systems), can be carried far longer. It’s a trade-off.
If one’s need for bird shots is only occasional, then getting that used/adapted telephoto lens can be worthwhile and so rewarding. It’s so simple a setup!
For example, I can, right now, use my existing high resolution 70-200 2.8L IS II with a X1.4 or X2.0 on my Fuji GFX, (which happens to share the same basic Sony sensor with the wonderful more established Pentax 645Z). Still, if I was going on Safari, I think I would bring with a second body just for distant work. In that case, there are many options, depending on whether or not onecrents or buts an additional lens to ones current camera collection.
My suggestion for MFT with either Olympus, as Robert awatchers uses or Panasonic, is because they are premium well-proven systems and as being so lightweight, make our MF bodies more carryable on a safari, where one has no assistant.
But most importantly, listen to Nicolas Claris’ advice and # 1, don’t waste money on a spotting scope. Used Pentax lenses are anyway optically superior in most cases and even less costly!
Asher