Nikon Z6 body from henry's Canada. $2199
Pentax K-3iii after year long discount $2249
For reach the Z6 with a 150-600 doesn't even match the reach of my DA 55-300 PLM with the 1.4 TC.
There are 31 S-line WR lenses available for the Z6.
According to
Camera Design
For Pentax
There are 10 weather sealed FF lenses tha use the same mount as the K-3.
9 DA*s all WR or AW and 9 DAs. Coming to grand total of 29 vs 31.
Of course there are many 3rd party lenses available, but for the most part, I consider 3rd party lenses to be stop gaps until I can afford OEM lenses.
The two systems are identical in almost every way, except to get the same results yogurt with the K-3 in terms of reach, you have to buy much more expensive, heavier lenses. The Z6 is only slightly smaller 100 gm lighter.
I should suggest your analysis might be a little biased. You do realize that the K-3ii has more reach with the 150-450 than the Z6 does with the 150-600, for very similar IQ.
The notion that you can get a better AW selection from the Z6 is problematic. There are more, but that doesn't mean you can't get what you want. The issue being not total lenses, but being can you get what you want.
Funny how the proponents of Canon and Nikon systems always go with "how many lenses are available" quantity, without reference to quality or the number of lenses you actually wish to own. If you are good with about 10 lenses and Pentax makes what you want, why does it matter if there are more available from Nikon, Canon or anyone else?
There are reasons to shoot any given system. Becoming a proponent for one probably just means you're blind to the benefits of others.
I like most others buy what's good for me. Comparing the K-3iii to the Z6, I'll go with the K-3iii, because 24 MP is action and birding gear, and the K-3 will get me more for my money. I may be true that the Z6 is good for others, but, we're all different, and when I look at it, I don't see a 50/50 situation with the tie breaker going to all the Pentax lenses I own.