Cheap 300mm telephoto: mirror lens, or legacy telephoto?

So I've been using my Panasonic 14-140 as my telephoto for wildlife, and while I really enjoy having the AF, 140mm just isn't quite long enough for useful birdwatching or other more timid animals. I don't do it often enough to justify getting the Panny 100-300, but if I could spend around $100-150 to get 300mm, I would enjoy having it around for those opportunities.

I'm a little bit torn between a lens like this Albinar 300mm f/6.3 mirror, (which, from everything I can see is a rebrand of the $250 Rokinon/Samyang/Bower version of the same) and a 300mm legacy telephoto, probably an f/5.6. My body is a GX1, so without IBIS it would probably have to be a tripod queen in either case given the focal length, but with the lighter mirror lens I could see using it in broad daylight handheld. I don't expect that would be reasonable with any 300mm legacy lens at all, unless someone has a suggestion for one in the 500-600g range? In general, I really would prefer a lighter lens, but if I'm leaving a ton of IQ on the table, I can compromise.

I've had some bad luck with legacy telephotos, specifically in terms of really bad chromatic aberrations, but I expect that's kind of par for the course. While purple fringing is usually pretty easy to correct, it definitely detracts from having crisp images, even if the colour is okay. I expect that's probably going to be the case with most legacy lens, unless they are very expensive apochromatic ones?

Anyone have thoughts, or experience with any suitable lenses? Thanks!