I'm curious about this now. I recall long ago someone starting a TAS on this and stopping because of desyncs. I remember reading through it and being amused that whoever was doing it seemed to be dreading the Dark Force/Mother Brain gauntlet.
You seem to have everything well in-hand here, but just in case, I'll link you to
this topic anyway; it's from when I attempted a run of the game on console. The important stuff:
-I finished the game with a party of Rolf/Rudo/Anna/Kain with levels of 18 across the board. I successfully run the Dark Force/Mother Brain gauntlet, though I do heal my HP before Mother Brain. Dark Force was straight up attack, Mother Brain was Snow Crown/Star Mist on the first round and go all-out on tech from there.
-I bought NO weapons/armor, only healing items and Pipes. Being a TAS, you should be able to forgo the healing items as well. Weapons/armor were either found in dungeons (Specifically dams) or stolen.
-I ran the game on the GBA version that was part of Phantasy Star Collection, but I don't think there are any difference from a speedrun strategy standpoint.
If there's anything else you'd like to know, feel free to ask.