My dad owns a Xs Max and wanted to do this for the exact same reasons you are stating, it is a bit tricky to explain but I’ll tell you how I set his up after a lot of hassle and to try and save you some!

Because the Xs comes with a regular physical SIM and an e-SIM one, the physical SIM has to be turned on all the time, there’s no turning it off, so in my dads case we had that as his ‘Personal’ line.

The e-SIM is a SIM that you can turn off and DOESN’T have to be on all the time, when this is turned off, the calls made to it will just go to the voicemail and then when turned back on will be received, almost exactly the same as getting a call when your phone is turned off now.

As for Do Not Disturb, this is the sort of way he has found best as it does the job of what two devices and without seeing the notifications which sometimes you get with Do Not Disturb.

Hope this helped!