I hope this helps
I have no experience with the K 1 or K 1 II
[ frankly I'm afraid to rent one because I then might be tempted to buy one ]
I use the K 3 and K 3 II with:
SMC DA * 300mm F4 ED (IF ) SDM 77 F
Tamron Adaptall-2 SP 500mm f/8 Mirror (55BB) 82 F
HD Pentax-DA 55-300mm F4.5-6.3 ED PLM WR RE 58 F
HD Pentax-D FA* 70-200mm F2.8 ED DC AW 77 F
HD Pentax-D FA 150-450mm F4.5 - 5.6 ED DC AW 86 F
HD Pentax -DA AF rear converter 1.4 AW
each has a place and purpose
with respect to the DFA 150-450, I use it hand held primarily but there are times it is a little wobbly [ a whole lot wobbly if the rear converter is used ]
I have used it with a tripod or monopod but mainly I attempt to use some of the techniques mentioned in this article:
Making the Most of Long Exposure Handhelds - Introduction - In-Depth Articles
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as far as carrying the weight, I have learned to carry it in different positions, slung on my hip with a hand grip being used, held cross body, held on a shoulder, to help compensate for the weight by varying how it is affecting my body