Earlier quoted context omitted.
Maybe a stupid question, but why? If you care about high resolution and good colors why are you keeping the low res second monitor? If you don't care about hires, why not simply set the Dell one to a lower resolution and not worry about scaling at all?
Because having extra screenspace to keep docs/chat/... on is nice and an old screen is good enough for that?
The thing is that scaling works just fine if all your displays use the same one. Two 4k screens -> awesome. Two fhd screen -> works just fine. It is only the combination of one hires and one lowres screen that is problematic.
So my maybe naive take away is that if you care that your main screen is 4k, then no your old screen is not good enough. And if you think it was good enough, then why did you feel the need to upgrade your main screen?