COR has to be the lowest level job you can be while being useful. thier buffs does not depend on skill and what not.
a cor can give attack roll at 14 just as well a 90 does, they get refersh at 40. If you where a cor and knew someone and pt was openmided you could be cor in a merit party as low as 40.
I think other people have said this but mage jobs always begain being useful for any event before melee. it would go bard being the second lowest level to do an event ot be useful, then healer, then magic DD if they where good, then melee, then tank. If people where open minded, the leech bard in a merit party could be as low as 55 ( the one that does ballad and cure support) the only issues with that is lacking curaga II, you would need 60 bard for that. Even though bards get march II at 60 and Valor Minuet IV at 63 the puller bard till needs 72~ to land elgy and Lullaby.
anyways a bard could be fully useful at event/ merit party as low as 55 while being 63+ would be perfferd.
for healer it depends, it could be as high as 75 for full shell and bar merits. at the very min I would say 61 for cure V.
DD nukers can start 65-75 depending what the event is. DD nukers where not even wanted for merit parties so hard to pin that on them. for abyssea you would need around level 70 for level corrlation and spells:
69 Firaga III
70 Water IV
71 Blizzaga III
for SCH nuker would just need to be slighty higher then a BLM like in the case of water IV it is 70 for BLM and 71 for SCH.
DD needs level corrlatation for melee acc and damage so it was useally perffered them to be 75. There was also gear you get to use as well as level corrlation to be useful. abyssea for DD may start around 70 depending atmas but i really think the exptation would be 75
for tank you need max evade skill/ shield/ level corrlation to evade hits easier and not to get hit as hard and so on. so tanks would be most hurt. so 75 or gtfo. tanks are not needed in abyssea party but I am more speaking on old ffxi to get across a point.
who can be lowest level > role that needs to be highest
COR>BRD>healer (whm)> DD magic> DD melee > tank.
hard to pinpoint RDM due to versatility. RDM depended on sub to cure so if you needed say.
anyways point is the issue in OP always existed, even back as late as 2003 when the game was first open to NA comp.
this issue is hard to "fix" and someting I would not put time in as a programer. it is just a side effect this game has with its job system. you are asking a bit much to "fix" this issue