I am not really so much of an "expert"; but I'll share questions and possible solutions with you.
Before the detailed questions: of the 150 or so photos that won't transfer - did you take these pics yourself? Or did you download them from wherever? Or did someone else send them to you? Vital questions when "Permission Difficulties" arise.
When you've answered that, we may be able to progress; if not relevant, then:
1. We need to know exactly what the "Recent Issues" were. Why? Because that precedes your problem and might, quite reasonably be the cause or related to the cause of your difficulties.
2. Do you sync your photos with iCloud? And if so, does iCloud sync with another Apple device; eg, iPad, iPhone or another Mac? Two separate questions for which answers are needed please.
3. Where on your Mac do the "thousands" of photos reside? For example: in the Photos app; in a Folder of your own creation; in several different Folders, each representing a year or a place you've visited etc.
Believe it or not, these are highly relevant questions.
Here are more:
How did you export these thousands? For example: via USB cord (probably), and tell us about the External Drive to which you transferred these photos: eg; Storage space when empty and how much left now after transfer?
Please remember
@Fsiwtfrr that we were not standing alongside you when all this business was going on. Hence the need for detailed answers
Recent issues: When browsing, I put Safari to the side to look at something else, then when I bring the window back in, it opens the PREVIOUS website for a few seconds then goes to the website I had open before I minimized. That’s strange, and not secure. I can no longer clone my mac unless I buy an external app when I used to be able to cli to an external in case of issues, but Apple took away my privilege to clone MY system. Several other less than positive changes over the last few years, but none of which have anything to do with photos. Most of the photos were mine, some were sent vis messages, some were downloaded, but many of each subtype did export. So it seems based on the questions that if I physically save a picture to my system on my hard drive, and add it to my library, I might not have permissions? Yet another “issue”. Icloud =yes, and my photos sync on all of my “i’ devices but 2 smaller older ones with insufficient storage. so 5, and my macs. Exported to a small usb drive, 500mb, total, 400gb free prior, then 379gb free after export. Exported photos were exported from the photo library, not copied from a different location.