Matching arm to cartridge is extremely important. Who or what gave you the idea the Triplanar would be better suited to the Miyajima, and why (i.e. make sure you’re not just going on price)? The Shilabe seems to be very low compliance, and the exchangeable headshells of the Ortofon would allow you the flexibility to add some extra mass if needed (e.g. 20g headshell gets you up to 24g effective mass there, vs. a fixed 14g on the Triplanar). You should definitely solicit opinions from Miyajima users, and what arms they like. But that Ortofon looks pretty nice as it stands - premium/custom Jelco build, dynamic balance, headshell flexibility, 12"...
Even if the new arm is higher quality in build, materials, tolerances, and overall design, it could all be negated if you're moving from a better marriage to a lesser one for the cartridge in question.
Even if the new arm is higher quality in build, materials, tolerances, and overall design, it could all be negated if you're moving from a better marriage to a lesser one for the cartridge in question.