You are correct on the Ring of Color Spray - it is a fixed HP that determines its effectiveness, not an intelligence based saving throw. I don’t know why I had that in my head the other way.
Eventually, I would really like my character to be a classic Fighter/Mage type. I always ended up there in BG1/2, either dual class or multi class. Not having played 5e tabletop, will a multi class fighter mage (after multiclass is implemented in BG3) be what I am looking for, or have the rules changed too much from 2e to 5e to get the same feel without missing out on too much optimization?
Short answer - we'll have to see.. long answer I pm'ed you since it was a bit off topic.