OSX Photos is Forcing Me to Optimize Storage With Plenty of Free Space
Two things to note:


the small print under "Optimize Mac Storage" states that "Originals will also be stored on this Mac if you have enough storage space", so hopefully the fact that you have plenty of space to spare will qualify as "enough storage space" and nothing will be deleted immediately (hopefully!)
even though you're not offered the option in the dialog, once you select "Optimize Storage", you can immediately switch the radio button back to "Download Originals to this Mac"

Neither of these things necessarily guarantees that the originals won't be removed and re-added, but at least you will end up with all the originals on your Mac once it has had a chance to synchronize everything to its satisfaction.