iMac Retina 5K, 27-inch, 2020 tiny storage

I was so excited to upgrade from a mid 2012 iMac 27" to this 2020 version with 3.8 GHz 8-Core Intel Core i7, that I did not notice I was downgrading from 1TB storage to 512GB!!! We thought; "we will swap out the hard drive for a larger hard dive - uhhhhh.....no, not an option without major dismantling this iMac.

Anyone have success upping your storage? Currently, I have a 2TB TimeMachine, 1TB External Drive and a small 512GB External Drive (not currently using). However, I had to place the iCould Photos Library, the Pictures, Movies, Videos, Music and a Documents (old files) folder on the 1TB External Drive. The problem is that this has made Photos pretty disjointed. I've been using technology since 1966, Apple products since my Apple IIe back in 1973, but this has me seriously scratching my head for a workable solution.