Yep. Deforrest Kelly died first. He was briefly in the first episode of ST:TNG.
James Doohan was on a full one hour episode of ST:TNG later in the series.
Nimoy was in two episodes of ST:TNG just after the movie Star Trek 6 came out.
I’m not much of a Trekkie, but my late husband was and I learned a lot from him, and followed the original Star Trek actors rather faithfully, I thought, until my husband passed away, and now, I just follow when the subject comes up.
The late Igor had a copy of the “Star Trek Technical Manual” or some such. It had patterns for the uniforms, a diagram of a space station and for a working Phaser. You might be able to find one on Amazon or eBay, if you care to search. It’s a fun read.