As a kid I remember having a split storage rules. All the nice (and at that time expensive sets) - airport, harbour, gas station, mini post office, mini hospital ... had to be packed separately after the play.
And then there was a lot of the universal Basic bricks and supplementary sets (roofs, windows,...) plus mixed bricks given by family. These were free to mix. We had red and blue such storage case closed by baseplates (really like those, see pic), one of the bigger basic set's box with plastic sorter and a big bucket to put them all in. Organization was up to ourselves and evolved over time.
Back to present. I'd say to keep the display sets intact - the ones made primary for display - those are the Creator Expert models, top SW models, Ideas and such. All the CITY , small SW, Creator, ... I'd consider as material for mixing.
To be honest, even a 5y old can make better buildings than the part and cost saving ones from LEGO if he has 2-3 spare sets. Maybe not some of the more complex vehicles, but the simple ones too...
Pic for old time's sake
And then there was a lot of the universal Basic bricks and supplementary sets (roofs, windows,...) plus mixed bricks given by family. These were free to mix. We had red and blue such storage case closed by baseplates (really like those, see pic), one of the bigger basic set's box with plastic sorter and a big bucket to put them all in. Organization was up to ourselves and evolved over time.
Back to present. I'd say to keep the display sets intact - the ones made primary for display - those are the Creator Expert models, top SW models, Ideas and such. All the CITY , small SW, Creator, ... I'd consider as material for mixing.
To be honest, even a 5y old can make better buildings than the part and cost saving ones from LEGO if he has 2-3 spare sets. Maybe not some of the more complex vehicles, but the simple ones too...
Pic for old time's sake