How to force optimize iCloud Photos library on internal drive with limited storage?

I am having an issue with my photos library not optimizing. I currently have my photos library location on a 2 TB external drive. I had selected to “download originals to this Mac” to ensure that I had a full back up of my iCloud Photos. I want to move my library location back on to my internal hard drive which has a total of 244 GB free storage. Question: is there a way to “force” optimization on my photos library so I can then move it over to my internal drive? Or do I have to create a new library on my internal drive with optimization selected and have the library build from iCloud? Also, does my iMac optimize the size of the photos library to how much local storage is available? (I am worried that if I do get my entire library optimized on my internal drive that it will slow down my iMac considerably). Thanks.

[Re-Titled by Moderator]

Original Title: Force Optimization of Photos?