@PhoenixSerenity @trendless I'd probably just put a Honeywell 50250 in each room they spend a lot of time in.
It's a workhorse, which has been in production for a good while.
It's highly likely to have continuous filter availability.
There's even third-party filters for it!
It's a little loud at full power, but that's only needed if there's a lot of dust, like from cleaning vigorously.