Thanks for the assist Colin!
After a rigorous search I just acquired a replacement canister filter (new Oase Biomaster Thermo @ approx. 45% discount). Compared with it's predecessor the long serving XP3, the Oase is filled with the same (or slightly more) quantity of mechanical media (sponges) but 3x the quantity of biological media (Biomax from old filter and 4 liters of new Matrix) so hopefully it will also be a step up in filtration. The pre-filter stack may facilitate routine cleaning (turtles are messy so that remains to be seen; I'll still be using an in-tank pre-filter over the intake tube) and the integrated heater is a nice touch. I think it also holds a good volume of biological media compared with the FX4?
It's more compact than the Fluval FX4 & fits inside my current cabinet so I'm not time pressured(!) to take on fabrication of a new cabinet. The FX4's bottom valve (though I have no floor drain only a sink) and the accessory Fluval gravel vacuum are nice perks, the FX flow rate is fantastic, and it looks 'burly' (but not sure about durability and reliability of design on FX vs. Biomaster).
Just got it hooked up yesterday (with much less than recommended water level) and so far so good, smooth and quiet.