Using just the skills and abilities available to us right now. This is what I would do to see SMN reach 6.0 where an actual rework could be done realistically.
Allow Devotion to be cast with Demi Pheonix and Bahamut out. - Currently this spell wiffs quiet often when trying to weave in enkindle and the assaults and leads to some of the "busy work" when getting ready to summon Phoenix as you stand there waiting until the egi decides it wants to cast the spell. If the Demis could cast it it would be less of an issue of timing everything if it could just be weaved between being summoned.
Remove damage penalty from spreading dots via Bane and increase the chance of resetting dots to 100% (including on the original target) - This directly increases damage (probably not on par with BLM by any means) and makes dot management easier on AoE pulls and allows us a quick way to refresh dots on single target fights. Skilled SMNs can possibly keep ruination spreading indefinably in a dungeon, and there would be zero excuses when bad SMNs let dots fall off.
Aetherflow given 3 stacks again. Energy Siphon usable out of combat to give you three charges, when used in combat it preforms its usual duty. Alternatively give it a long (4s) cast time and it gives you 3 aether every time its used and has no cooldown, allowing the SMN to stack up out of combat and gain aether during transitions. Cooldown independent from energy drain so it would not supersede drain during normal rotations. - This opens up aether usage to be far more reliable, allowing pre pull preperations, and makes downtime in fight feel like a boon.
Egi Assaults should instantly refresh when a new egi is summoned. - This allows titan to actually be used as a defensive cooldown when needed, and allows 4 stacks of ruination to be built back up during that downpahse after Pheonix and before Bahamut trance is back up.
Deathflareshould be guaranteed Direct Critical Hit - This is a damage increase obviously, but makes Bahamut trance feel good again, as that huge number was one of the perks to pulling it off pre ShB.
Demi-Bahamut should be rooted in place to ensure wyrmwaves go off, and the duration increased 5seconds so three Ahk Mourns can be used again.
Physick, scale this off INT or make its potency worthwhile - This is the only defensive thing we have when titan isn't out or has charges. Increase its mana cost to 2400 if you must to avoid people actually using it in a meaningful way, but for the love of Menphina make it do something.
This is all I feel would be necessary to get SMN back on track. No i don't like the current summoner, No i don't think it can be salvaged with the current toolset. but this is atleast attainable with what we have now.