I prefer the separate box setup to some extent, but for only one reason. That you can reboot the separate router without upsetting the sync/DLM.
But how often other than when experimenting do you need to reboot the router? If frequently, then there is something wrong which needs resolving. (Though on FTTC that could result in DLM screwing you up for months).
I find most reboots are of the modem, either standalone or as that component of the combo.
Having said all that, I can quite understand and accept what you are doing with your setup. A setup that to me suggests the two-box modem+router is a good idea. But your usage description has to be wayyyyy out of ordinary

.
Leaving my two points. For the vast majority a single-box solution is fine, adding a WAP to it possibly needed a fair bit of the time. With the second point that the single box provided by ISPs, (and - note! P & P charged for even if the box is "free"), is normally rather basic. In ADSLx days modem/routers such as the Netgear DG834 series were pretty good things to have.
Kindness isn't going to cure the world of all its awfulness but it's a good place to begin. Daisy Ridley.
My broadband basic info/help site -
www.robertos.me.uk. Domains, site and mail hosting -
Tsohost.
Connection - AAISP Home::1 80/20. Sync 54999/14466Kbps @ 600m. BQMs -
IPv4 &
IPv6