I wouldn't think a Pro-Ject would be an upgrade over the MMT. The SME V or 345 are too dear at this time and probably overkill for an Alexandria. The Wand+ may be a great option.
https://www.oracle-audio.com/oracle-paris-tonearm
I apologize, the tonearm I was thinking of is for the paris see link above.
It seems that your Sumiko arm is more or less a modified Jelco 750D with a longer overall length (a cursory search gave me this info).
Definitely check out the wand and compare it to some other arms, I don't think you'll regret the purchase.