Try a Yamaha reface DX, its portable and also has a killer keybed. Or even consider an og used DX7 or variation on the 80s yamahs fm lineup. Since your looking for melodic pieces. 16 voice fm and velocity plus aftertouch is way better for performance. I've seen many used dx7 around 500 and reface dx is 280 new.

If its only volca fm vs MC comparison, I would probably go for MC if you can wing it moneywise. Even if its 2x the cost. The feature set for the MC is amazing for the price. Best sounding groovebox for the money. As the necessity for midi controller, I would def need one if ur using the volca. The volca has no velocity only an unoffical firmware update provides one or you can purchase a retrokit rk002 smart midi cable (additional $50), but would still need a midi controller.

The MC has atleast 6 velocity pads as well as a 3 note chord machine so atleast there is an option for you there.

I guess if your looking for portable small groovebox MC is a no brainer, but a Yamaha reface dx would be my choice if your looking for an actual dedicated phyiscal FM synth that is portable and its main use is for melodies.

Also you might look into vintage yamaha portable fm synths. After their successful dx7 line, all of their small portable consumer music keyboards were fm derived synths. The used market is a great place to find original sounding synths that most musicians ignore.

Also, also if you find a deal try looking for a casio cz 101. Casio's Phase distortion (PD) synthisizers are crazy fun and inexpensive too.