For reference, the machine in fourth place on the comsol performance leaderboard thread is running almost the same power level (500 watts) and is cooled by a single 360mm radiator (the dual 180mm variety, so ~50% more surface area than yours) and is hitting 77C temps @ 25C ambient, so I don’t think there is anything inherently wrong with what you’ve done (like bad TIM or insufficient mounting pressure).
I’m not a fan of remote mounted radiators because of the extra failure points they introduce, but if you do go that route I’d suggest getting quick disconnect fittings on the tubing so that it is easier to move the machine around; I believe koolance is currently making the best QDCs.
While MORAs are convenient in that they are mostly pre-setup for external use, they use old fashion round tube radiators instead of modern flat tube radiators which makes them perform noticeably worse than any other modern radiator of a similar size.