Hands down PERSONALLY i would go the Z9 every time over any other Z product,
and here is why
If weight and size is not an issue and you don't feel another Z8 is exciting you, just spend the little extra money and get a Z9 for the experience if nothing else, you deserve it anyway. RENT a Z9 FOR A DAY................its likely you wont give it back LOL and if you buy one the rental cost comes of the purchase price in cases.
The advantage of the Z9 over the Z8
The Z9 by comparison is just a beautiful feeling camera and has clearly a lot to offer over a Z8 and yes especially durability if that's on your list.
If you look at the inner workings of the Z9 compared to the same workings of the Z8, the Z9 is encased in a significantly larger robust carcass also acting as a major size heat sink, again compared to the conventional consumer vulnerable sized body of the Z8 that while having excellent video performance, logically it has video limitations compared to the Z9.
I love the size and weight of the Z9 for balance comfort and stability hand holding again other than weight and size for travel or hiking.
The built in grip that is so superior to the add on version for the Z8, also the Z9 its such a joy to use, again the feel and fit in the hand, size weight ergonomics, you wont want to go back and use anything else, it has the extra assignable button, far longer video recording capacity with no overheating limitations, and above all it has a much longer lasting battery, and the same two CF slots.
So yes in the real-world its a better camera user experience, most owners i know who have a Z8 and Z9 combo seem to relegate the Z8 as a back up and travel tool making first choice use the Z9.
If you can wait a) the new Z9 II isn't far way b) there may be some newer options Z6 III Z7III
however with the recent price increase of the Z8 Z9 it will make the Newer Z9 no doubt the Z6III Z7III etc more expensive.
It also depends on what you do need or really want, for myself in your position i would rather focus on critical high use need glass than another body, unless your happy with what glass you have.
Again for me in the Z range its the Z9 100% over any other Z product, if weight and size is not an issue,
that said after 2 plus years i sold my Z9 (out of warranty) recently to
a) preserve equity while i could.
b) i picked up a new bargain Z8 with full 2 years warranty, before the price rise.
WHY .............only because what i do has recently changed where weight and size was becoming a major factor, which is also why the Z8 was i feel really created,
also i don't do video, i now go more on really long adventure hikes as well as travel much more, also i now do more street nature and public environmental photography, the Z8 fits that tool need in spades.
However I am waiting to see what the Z7 III brings in terms of speed resolution and ZF features, if those tools suite more of what i do with tangible advantages i will sell the Z8.
I also have transitioned from owning a lot of expensive gear to simply much more renting - borrowing as i don't have a lot of high demand need - use for one specific tool or dedicated application any more like wild life sports action etc, now do a little more of everything more often.
When i need to do wild life or surfing or sports action i often rent a Z9 or mostly a D6 usually with a 600 F4 FL or what ever else i need, so nice to hand it back.
If i had a expensive exotic mirror less 600 F4 TC with a Z9 on the shelf it would need to get dusted off in between uses.
Only an opinion