Summons shall be supporting the gameplay, not BE the gameplay, imo.
On the other servers I played, they were always much weaker than the actual PC, and I think it is better that way. For it makes you want to group up with people. On arelith, I don't really see the reason why. If you summons are strong enough to do the end game grind dungeons and/or bosses, there is no invective to invite someone else, because you would just have to split the rewards.
Also I don't see reason, why should be some gamestyle (summoning) be more rewarding than other gamestyle (like sneak dps). One can do the end game boss, get the sweet reward from chest, the other will have problems to even get to the dungeon solo. One will never need to work with others, and still get the advantages of end game loot. I believe (but I don't have any data) that vast majority of solo dungeoneers (epic loot grinders) are some kind of summoners.
But even if you really want this to be the standardized gameplay, this is still not good solution.
I don't even know where you want to go. On one hand, you improve the spells for summon buffers (mass animal spells, stone bones made aoe - which also is bugged and buffs even enemies), on the other, you take it back. Pendulum without the vision.
Interludium: The sequencers are abomination.
I really don't get this solution. It is sloppy, not thought through, and goes against the setting logic. To be the best summoner, you don't have to be a good caster/buffer. You have to be a craftsman and spend time getting resources for crafting. This is boggling.
Why is the mundane character as good at buffing his summon (exampla gratia bg) as 30 levels of wizard, cleric, druid et sim. casters?
Why do I (as caster) need to bother with buying some **** from mundane characters?
My logic would go the other way. Make base numbers of summons lower and only when the caster buffs the summons, they go to usable level.
Why are there even these sequencers? To make the buffing faster? So just make sequencers like the sequencers should work. Item, on which I cast the spells, and when I use it (interruptible) out of fight, it will deplete casts from my spellbook and it is done.
To give casters feeling they are useless at buffing? Mission accomplished.
But back to the topic above. If you really want group of people, who play the style: log, go to the epic dungeon, click on summon spell, wait until summon gets to the end, loot, log off; there is still much better ways to do it. For sure, I would get rid of the sequencers and lowered the basic numbers. First that comes in mind is of course give summoning bonuses (like for the epic caster) for pure caster. Easiest, probably effortlessly applicable to the current state.
Lvl 21: +1
Lvl 23: +1
Lvl 25: +1
Lvl 27: +1
Lvl 28: +2
Lvl 29: +2
Lvl 30: +2
Then you will have PvE gods, who doesn't even need to be mentally present, but on the other hand, they will not be much threat in the part of the game, where it matters, which is PvP. And are generally limited.
Adding (or changing the generally useless) lower tier spells, so they contribute in summoning and give advantage to summons in some way. This will hurt spontaneous casters with limited spellbook without the option to swap. But again, you can be more creative here, like spells, which when damages enemies heals their summons, or on kill give short time (minute or so) buff to them. The gameplay doesn't have to be cast summon, use bought item from shop and spam cantrip.
Other way is to make make prestige class 5, 7, 10 lvls, which will focus on summoning, have bonus features from skill leadership for summons and many other things that people who are using summons can come up with. Not me. (Also, with the number of bugs and everything, I would say don't add new class for at least year, until all of the previous are repaired).
P.S.: Funny what half an hour of procrastination will do, I don't even have stakes in summoning, I am not currently using it, and I don't generally care about it as long as it does not effect PvP.