Good mid-game monsters? (spoilers)

Slime Knight and Orc King, very much. They'll be fine through the endgame and bonus material. Cureslime will work; if you chance to get a King Cureslime when you find them later, he's better, but he's 1/64 so not easy to get. Cureslime is OK and has that all-important omniheal eventually. Drag-goof is probably the least built for the long run but will last you for at least the short term, and Hoodlum is somewhere between drag-goof and the rest. If you don't want to grind for anything, probably the only thing left that's an easy recruit is Golem, who will turn into a tank a shade better than hoodlum.

Also, if you JUST reached Gotha, then not too long from now at all you'll have another major story event followed by an influx of a few new character choices, a couple of whom you'll want in your squad. You'll see what I mean very shortly.
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Frazee is barking weirdly!

I assumed with my post that the influx of human choices in Gen 3 Gotha would have been a spoiler, but apparently not. I also assumed you didn't care to grind any 1/64 choices. If you do want to grind for some 1/64 choices, that opens up a few more possibilities. Archdemon in the mantleplace, Great Dragon in Nadiria, for sure going after that King Cureslime in the tower to Zenith, and of course for somebody who really wants a goofy challenge, trying to get as many of the Pips and Conks as you can. The non-magic-using Pips and Conks actually become perfectly viable because they level into decent melee tanks, though nothing on the order of a Golem, but the magic-using ones will require tons and tons of seed farming to make viable.
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Frazee is barking weirdly!

One that you're already well past being able to get is Chimaera. It's a mid-level difficulty, though I forget right at the moment whether that's the 1/32 level or the 1/16 level of mid-range for that specific one. It has pretty good utility, has breath attacks, decent weapons, levels to L60, and can be a secondary healer, so I've always kind of liked that one. Also, its sprite is a DQ standard all the way back to the very first NES game. It can be found south of Mostroferrato and in and around Mt Magmageddon.
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Frazee is barking weirdly!

I've never significantly leveled a Samigina, but I can see the potential for it to be exactly that...
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Frazee is barking weirdly!

Yeah, in my experience the pass to Gotha is the best place to hunt the liquid metal slime, and getting three samiginas and three minidemons to remove their recruitability from the equation can be part of that process. My first file, the one I have all the recruits all done, I had two samiginas and three minidemons when the LMS stood up, if i remember correctly, and released the spare mindemons again but kept the second samigina. I just never got far enough in my list on that file to level up either of the two and try them out.
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Frazee is barking weirdly!