Where could my photos from 2016 and 2017 be?

I am currently going through my old iPhone photos on my Mac and have noticed I am missing April 2016–August 2016, and October 2016–May 2017. To be clear, I've got everything pre–2016, and everything post–2017, so it's strange I have these missing patches?

We had problems with our Mac running slowly at some-point later down the line, in which case everything went on an external drive for us to wipe it. I'm worried I might have accidentally deleted my Photos Library during this process, or something went wrong there.

Some of the photos are in iPhone backup files (which I believe I have located all of them), whereas others were archived simply as files in folders on the external drive. My external drive is full, and very complicated to navigate, could they be located in encrypted backup folders like iPhone backups are, in which case, how would I locate these?

I've managed to go all this time without noticing that they are missing so it's not a massive issue, I might be able to string some of them together from where I've shared them with friends in Messages etc.