Seems like the overwhelming consensus is either a D500, or wait longer for a Z8/9. My only concern about the D500 is its lower resolution compared to the D5600, when I'm already a little concerned about IQ, but perhaps the better processor and AF will balance it out and the IQ would end up comparable or improved.
@jcollins it's not incredibly urgent. Even if I go with a sub-$900 used D500, I wouldn't upgrade for a few more months. I'd still use my D5600 after an upgrade for macro with my 105mm because of the built-in flash. You raise a good point about how long I'd be satisfied for. I'd probably want to transition to mirrorless eventually when I have enough money to do so. Maybe I should hold off on an upgrade until then, or maybe an upgrade to the D500 within the next 6 months is fine since it's a far smaller monetary investment than a Z8/9 and the AF capabilities still hold up well to this day.
@Steve W I'm not really concerned about having to shoot in f/8 since that's what I normally shoot at/around anyways. What I am concerned about are reductions to AF and IQ, especially since the D500 has a lower resolution than my D5600, so any additional loss in IQ would be noticeable. However, the 1.4X TCE would be the cheapest way to surpass the resolution of the D5600 with a D500 and 200-500mm. Should I even be concerned about the D500 being 20.9 mpx vs 24.1 mp?