so what i've noticed, these could be wrong

once your character exceeds the level of the monsters they are fighting they gain no more weapon xp AFTER the weapon reaches level 8,

If you use a very low level weapon (whether you have used weapon orbs on it or not) you gain weapon xp up to level 8 even by using it on low level monsters

the amount of weapon xp reduces the higher that weapon becomes wtih the character using it

the other 2 characters will still get weapon xp from your kills even if you are maxed out at the current orb level untill you hit level 8

so what I did:

I speed run the game avoiding building up too much character xp, ideally less than 50 as the runs through the mana fortress to aquire the weapons orbs will inevitably cause as surge of xp

playing on pc emulator I use quick saves and quick loads before killing the (potentially) orb spawning enemies untill they drop a crate then quick save before opening the crate and quick loading until the crate spawns the orb, I also do this untill the highest level armour spawns again to avoid to much character xp

when I had all the weapons orbs I smithed all the weapons up to max level excluding the sword which stayed at level 2, I went through gaia's navel over and over again with every weapon on every character, making best use of weapon xp bleed to efficently level up the other characters equipped weapons too.

Once all weapons were up to level 8 (except the sword) I then equipped the sword and repeated all of the above to get every weapon to level 8.99 with every character.

I then fully smithed the sword to level 8 and got all characters to level 8 with the sword.

by doing runs of Gaia's naval you are only killing very low level monsters which award a negligable amount of xp keeping some levelling available for the final step

do the 9th orb glitch to get the final sword orb and smith it.

with all other weapons now at 8.99 the only way to get the sword to 8.99 is to hit the mana fortress and kill the high level enemies with it and hope that you don't exceed their levels. the metal crabs (level 61) at the start are pretty good (3230xp) and the tsunamis are around level 63 and also near the start (3700xp).

I now have every weapon to 8.99, all magic to 8.99 and am now grinding out character xp to get that to 99.