Us Pentax shooters are an endangered species. I chose Pentax mostly because it just felt better in my hands than a Nikon and way better than a Canon. Also, it (Pentax K50) was comparable to the Nikon d5200 but priced like the d3200. Basically a mid level camera at an entry level price. This was in 2015. Now, I'm bought into the system and at my age, will probably never switch even if Pentax never releases another camera. Mainly because what I have is plenty good enough for me. However, they do have a new flagship APS-C DSLR (Pentax K3iii) coming out in a month or so, and another DSLR of some sort in development.
The 300mm is a great top line lens. The photo of the Trumpeter Swans I posted last month was taken with it. But, it can sometimes be a challenge to find a subject where it would be the right use of it, especially at a zoo. It's also too heavy for me to carry all day long, but I do love using it. The 55-300mm is great for zoos. Not extremely heavy and covers just about everything you need, except maybe some of the close up indoor exhibits. When I go hiking I keep the 16-85mm on the camera and the 55-300mm in a belt clip case or in the backpack. It's a great combo for me. I'm also thinking about adding a 100mm (or thereabouts) macro to my kit. Just not sure I would really use it enough.