Congrats on getting so far in the game, and for giving such well thought remarks.
Up until today, I had a 170 hour saved game, which was deleted by my little cousin. I'm still aching from it, but I'm starting over again. I agree when you say that this is the best DQ in terms of value.
Some pointers. If you go to the Tower of Nod, starting from level 3 and upwards, there's a tiny chance that any enemy will have a Metal King Slime with it, making it the best non-grotto place to level up.
MKS appear in Water type high level grottoes (Lvl 60 and upwards), these Grottoes usually have Moor or Lake in their names.
Platinum King Jewels give the highest amount of exp, but only appear on the highest Lvl ruin type grottoes, which are the most common kind (Lvl 80 and upwards).
I had the Luminary class as well. Moai Minstrel appears on Lvl 60 and upwards Nature type grottoes (these are also the most common kind, they look like a tunnel).
Current PC Build
CPU - i7 8700K 3.7 GHz (4.7 GHz turbo) 6 cores OC'd to 5.2 GHz with Watercooling (Hydro Series H110i) | MB - Gigabyte Z370 HD3P ATX | Gigabyte GTX 1080ti Gaming OC BLACK 11G (1657 MHz Boost Core / 11010 MHz Memory) | RAM - Corsair DIMM 32GB DDR4, 2400 MHz | PSU - Corsair CX650M (80+ Bronze) 650W | Audio - Asus Essence STX II 7.1 | Monitor - Samsung U28E590D 4K UHD, Freesync, 1 ms, 60 Hz, 28"
