Why can't I add more vector layers without QGIS freezing up on me?

I’ve been working on a project for a quite a while, I’ve loaded many vector layers, including several hundred short line segments that are kmz files, as well as topo lines, lake and glacier polygons, rivers, roads, vegetation, hillshade, etc.
I’ve also created about 50 Map themes and map layouts that use these layers in many different configurations. Everything is quite intricate and has taken a ton of time to set up.
Unfortunately I’ve run into a problem where I can’t load additional vector layers in. When I try to add a layer, the layer will appear in the layer panel, but the program will then freeze with a spinning wheel. Generally if I click a few more times QGIS will alert me that the program isn’t responding, and I restart. Once or twice I left it for a long time, maybe 30 minutes, and it seemed to resolve and was usable again, but then froze up in the same manner shortly after.

I’ve tried trouble shooting and found that if I remove about 80 percent of the kmz files the project seems to function normally and I can add new layers, however this won’t allow me to finish my project. Also, I was able to continue to add new kmz files fairly recently so I don’t’ understand why I’d need to remove so many files for it to work normally. I’ve tried to determine if there’s a corrupted vector layer by removing the layers in different combinations but it doesn’t seem like there’s one file causing the issue.

The other possible problem I’ve tried to look at is whether the map themes could be causing an issue, perhaps if they link to a layer that I accidently deleted, but this doesn’t seem to be the case.
Any thoughts on what might be going wrong?

One thing I just noticed while using the plugin Layer Board. When I use it to look at the layers, it identifies over 1200 layers. As part of my workflow I’ve been editing my vector layers in Google Earth and saving them as kmz for import as vector layers into QGIS. Later I often tweak the layers in Google earth and overwrite the kmz using the same name. those files edited vecor files show up in QGIS in their updated configuration without having to add them again. However, looking at Layer Board it looks like all of the previous kmz’s that were overwritten are being considered as vector layers in the project. Most vector layers show up several times in the Layer Board list – The layer names are the same, but the IDs are not the same.- the number/letter code added to the name for the IDs are different. Instaead of tracking about 350 kmz, it’s tracking over 1200, more than half of which were overwritten by a file with the same name. see the inlcuded screes shot to clarify.
could this be causing my issues?