Using external SSD as boot drive
I have a Late 2013 21.5in iMac 8Gb with a (slow) 1Tb 5400rpm HDD running Catalina. It takes a good 5 minutes to boot and another 5 minutes to settle after signing in. I like to use Lightroom Classic and Photoshop to process my photos but the load time for these apps is very slow for Lightroom and impossibly slow for Photoshop to the point that I have considered purchasing a new machine. Mojave was always slow so I hoped Catalina might improve things but not so, which made me suspect the HDD to be the source of my problems. I would like to avoid downgrading to Mojave if possible.
However, it is in my mind to purchase an external (Seagate) SSD 500Gb, clone my current main HDD to it using Carbon Copy Cloner and use it to carry the system and apps for booting. When things have settled down I will then remove Catalina and then use my current HDD for data only.
I use Time Machine and Onedrive as backups just in case of problems.
Does this plan sound reasonable?
iMac 21.5", macOS 10.15