Best times to visit parks?

I can not comment too much on birds. I think many birds breed just before the wet season, so that seems to be a good time/ easy time for photography. Great Hornbills in Khao Yai start earlier, January/ February, with nesting. And some will be nesting until May.

I love the wet season. Not sure why everybody always seems to say there is less to see.
Maybe that is the case if you sit in a hide at a waterhole which is less likely to attract wildlife because water is everywhere.
Personally I prefer actively searching for wildlife, instead of passively waiting, and see at least as much mammals in the wet season as I do in the dry winter season. Honestly I think I see more. Surely if it is raining it is hard to see much, but it's not commonly raining the whole day. And as soon as the shower passes, the wildlife will be more active.

I guess you are right that for photography it is not an easy time. Low light, low contrast. Though the latter can be better when shooting in the forest.
Too much sun means harsh contrast.
For insects, reptiles and amphibians nothing beats the wet season. Haven't been out much, lately. Just only last Sunday some herping with another member of this forum who wanted to see some snakes.
The morning was sunny, so within 5 minutes after entering the park we already found an orange morph Oriental Vine Snake hanging in the bush on the roadside.
The rain was indeed a problem, but after the rain the Large-eyed Pit Vipers that we found near Haew Narok were still in the same ambush position like we found them an hour earlier in the rain. We did not search for mammals, was not the aim of the trip. But on the way back a big herd of elephants was on the road. I guess you might have seen them in the news. I am not sure if the incident happened before or after us driving by.
Glad the guy on the motorbike did not get killed. GOod lesson to us all. Just Wai to the animals and you will walk away alive. All they want is respect ;-)

The day before, and that same morning elephants had been seen in various spots.
This year July - August was very poor for elephant sightings in Khao Yai, I presume mostly due to the long drought.
I guess they moved to different parts of the park, possibly lower elevation to find water.
Glad to see they are showing up again.
Also talked with another photographer that was in the park and told about a fruiting fig. He showed me his images of about 10 Great Hornbills in that fig tree. I guess living near the park you have seen enough of these ;-) but it's just to show that if you know the right spot, you can still find the birds.
There are enough resident species that not suddenly disappear in the wet season. But indeed at some point after nesting/ mating season, they might be less vocal so a bit harder to find.

Good time or bad time, just go out there. You will see more than when staying at home ;-)
Most animals have nowhere else to go, they are around the whole year round.