You've run some pretty fancy MC's, but the _______ MM cartridge really impressed me

 Nagaoka are superb by reputation. (I’ve not heard one.) Their TOTL MP500 can be had at the low end of your price range. Personally, I have been consistently underwhelmed by Sumiko, and Clearaudio cartridges are typically made for them by someone else, which raises their cost compared to similar performers. Among the 3 brands the OP named, I’d go Grado.

Another Curve Ball, I met with a guy at a all weekend event where the Public Attended to receive demonstrations of HiFi Enthusiasts exhibiting their systems.

I was introduced to a Phillips Ceramic Cart' and was so impressed with what was happening, revisited the room multiple times and had Tracks from my owned Albums played for me on a few occasions.

I learned that the Cart' user had stopped using a very expensive MC Cart' to make way for the Phillips Ceramic Cart'. I also learned that when the Ceramic Cart' has a Paratrace Styli added the Cart' owner had decided to stop using MC Cart's. 

Following the encounter, it took a few months of continuous searching, but I was to be  extremely lucky to find a exact model Phillips Cart and a spare new Stylus.

I have spoke with a Third Party Rebuild Service who are to carry out another build for me. They feel confident that the Cart' that is to have a Boron Armature removed , can have the Armature repurposed with the addition of a new Styli and exchanged with the Stylus supplied with the Phillips Cart'. I will then have a bespoke Cart', as well as a option to return to the OEM design.

 The plan is for this Cart' and other owned MM Cart's to supersede my use of MC Cart's. As an initial set up, there is the intention to run two TT's, one dedicated to the use of MC's that will at some stage no longer be replaced, the other TT will be for the Ceramic and MM Cart's.

There will be an interesting period making the time to revisit the stored selection of owned TA's to see where the Ceramic/MM's can be discovered to produce the best SQ to suit my tastes. I am working on the basis a dual TA may need to be designed for the Plinth, as there might be a need for this to get the better out of the different designs. For too many years now, all efforts made have been about discovering where there is betterment to be found for the MC Cart's used. 

A new TT acquisition has been made today, which is the same model as my preferred TT, as the initial preparation to get this experience underway. 

Pindac, I think the idea of a ceramic cartridge in this day and age is more akin to a screwball than to a curve ball, in baseball parlance.  This is not to say it's crazy (one definition of screwball) but that it is more esoteric than a curve ball.

Everyone always forgets Goldring. The 1042 is an amazing cartridge for the money. It is the least expensive cartridge I know of with a Gyger S stylus. My other favorite HO cartridges are outside the OP's budget. 

Soundsmith makes great sounding MI cartridges, I'm currently using the Carmen 2 nice all around sound and very ready rebuild prices.