I don't own the K-1 or K-1II, but I do own and occasionally use a Hasselblad HV. It's based on the Sony SLT-A99 full frame camera, which is very similar in size to the K-1II - just a little wider, but very similar height, and slightly thinner. Importantly, it's about 15% lighter than the K-1II. The lenses for it - whether compact traditional primes, variable or constant-aperture wide-to-short-tele or telephoto zooms, consumer or pro level - are all similar in size and weight to their Pentax equivalents, as you'd expect.
For several years before acquiring my HV and lenses, I'd been shooting primarily with a Pentax K-5 or K-3 and mostly dedicated APS-C format lenses. Switching up to full frame with the HV, I really noticed the difference in size and weight, especially when walking around for a few hours (and with early-onset rheumatoid arthritis starting to kick in gradually). At home, or stepping out of my car for half an hour, the size and weight aren't an issue, but a full morning or afternoon's shooting whilst walking is a different matter, and an entire day lugging the full-frame camera about with a 24-70 f/2.8 fitted and 70-200 f/2.8 in the bag is a chore for me; do-able, but not a great deal of fun.
Really, I think it's the lenses that make the most difference. If I just have, say, my old Minolta 50mm f/1.7 on the HV, or even a variable aperture, light-weight consumer-level zoom lens, I could happily carry that all day... but a fast, constant-aperture 24-70 or 70-200 zoom is a pretty bulky and heavy thing (according to my own tolerances, of course), and combined with the bigger, heavier camera, I personally don't find it enjoyable to carry around for long periods - especially if photography isn't the only objective of my day.
Given the above, I decided some time ago to stick with Pentax APS-C for my day-to-day shooting, and I only bring out my full-frame kit on select occasions. I'm glad I own it, and I
do use it - but far less frequently than I'd anticipated.
Since you're used to shooting micro 4/3 - mirrorless systems with thinner bodies (due to lack of mirror mechanism) and smaller lenses due to the cropped sensor size - I think you'll find the K-1II a quite bulky, heavy camera by comparison (especially if you already found the KP heavy), and more-so once you attach faster lenses to it.
There
is, however, a "look" to full-frame images that sometimes - depending on the lenses and apertures used - can't be easily achieved on anything other than a full-frame system, and rarely on micro 4/3. The shallow depth-of-field available at moderately wide fields of view and anything tighter can appear remarkably three-dimensional and pretty, with a real sense of "space". This offers some nice creative possibilities, depending on the use-case. For example, shooting a 35mm f/1.4 lens wide open on full-frame creates remarkable separation for the given field of view... On APS-C, you'd need a 24mm f/0.9 lens to achieve a similar look. Such a lens doesn't even exist to my knowledge - and if it did, it would be bulky and expensive. In micro 4/3 you'd need an 18mm f/0.7 lens
As @robgski suggests, it would be advisable to rent a K-1 or K-1II and a couple of lenses at the focal lengths that would suit you - or, ask around the forums and see if you can find someone relatively local to you who would meet up so you can try their camera and lenses for a short while, just to get the feel of them.
EDIT: Whilst I found that APS-C format gear is my "sweet spot", there are days - an increasing number of them, actually - where I don't even want to carry my APS-C kit around (for a while, I considered a micro 4/3 kit for such days, but I'm already maintaining two different systems - and that's more than enough ). If photography is very much secondary to my reasons for being out and about, I'll often take my little Pentax Q7 and a couple of lenses, or even just a smaller-sensor compact. As always, it's about picking the right tool for the situation...
Last edited by BigMackCam; 05-02-2022 at 01:36 PM.