B1,000 currency notes
 

I still have a bunch of 4-year-old B1,000 bank notes with Bhumibol's image; are these stilll valid?

Yes those should be perfectly fine - I get those very often. There are currently (at least) two "generations" of 1,000 baht notes in circulation, the ones from the King Rama IX era as well as the new ones with Rama X.

I prefer the older ones with the various age portraits.

Most definitely acceptable. I used several such notes today (October 2022), earlier this month, and last month.

They are becoming more rare however. Most of the bills in passing through my wallet these days feature the newer monarch.