Advice...

[Karshan]

I hardly think its me.

Any Summoner is going to be hard pressed to keep up 630+ average potency of single target attacks unless they're playing absolutely perfect with no DoTs falling off and always hitting every trance in perfect timing.

That said at present I'm only 77th on both Jobs SMN/RDM as I go through Shadowbringers and do not have Phoenix Enkindle yet, but I don't think its going to make much difference.

630 on 2 GCD (5 seconds), so just 126 pps on that GCD "basic" spam. The fact the second spell goes instant just means it goes off at the beginning of the 2.5 seconds, not that it doesn't trigger the 2.5 seconds before launching anything else.

In the same time span of 5 seconds, also with the "basic" stuff a smn does :
- 2x200 GCD (ruin 3) = 400
- 5/6 of 2 ticks of both dots so 0.8*2*(50 + 50) = 160
- 5/6 of 2 pet autos so 0.8*2*(80*0.8) = 102

662 in total. So absolute basic spam for both does the same approximately.

If you want to know on a more global basis, RDM global pps, counting everything from melee to oGCD is around 205 while SMN is 220 (while both correctly executed). You cannot fall behind RDM as a SMN is both are executed well.

EDIT : because it's important ; I failed to pay attention to your last sentence : it is completely false. Like it was in Stormblood, SMN gains a massive bump in potency come the last level.
Indeed, between 72 and 79 it goes like that : Baha trance > Bahamut > Phoenix Trance w/o Phoenix > Repeat.
At 80, you add 5 second to the Phoenix Trance and, well, Phoenix, which is a Baha like.
It would be close from one another if during 72 -> 79, Phoenix trance allowed you to summon Bahamut after it, but it's absolutely not the case. So Phoenix is a complete gain at 80, twice the Demis in the same timespan. More gain than Scorch.