I'd argue this seems like the most natural upgrade of the new SMN and is a fairly realistic prediction, however it does have an annoying flaw. Having a 2 minute rotation is something SMN has had in the past and a big issue with it is how punishing it makes getting KOed. If your Leviathan, Ramuh, and Shiva phases are stronger than your Ifrit, Titan, and Garuda phases, then a reset on KO will feel really rough. Now, this doesn't mean it's entirely bad nor that it can't be done. Like I said, SMN had this before in SB, but it's definitely a negative that needs to be looked out for.

Here are some general thoughts I have about the long term future for SMN myself:

1. I'd like to see a fix to how pets work so that Carbuncle is no longer dismissed when summoning Demi-primals. The only reason he's still dismissed is because of old code, and while I really can't speak to the difficulty of making that a reality, I think it's very important if we want Carbuncle to be more than window dressing.

2. If point 1 can be done, expanding on Carbuncle's repertoire of tools, such as returning Searing Light to him. A couple other tools that could be associated with Carbuncle that I think would help keep SMN's rotation feeling fluid would be an action that grants you a random Gemstone when you otherwise have none, useful for delaying your next Bahamut/Phoenix in dungeons without just spamming vanilla Ruin/Tri-Disaster. You can only use this when you have no Gemstones to avoid exploiting the action to repeat your highest damaging phases. The other action deals very high damage based on your remaining gemstones and consumes them. This should not be as strong as the entire gemstone phase itself, but useful for when you're unable to use all your Gemstones before your next Bahamut/Phoenix, such as when a boss becomes untargetable for a segment of the fight. These may need to share a cooldown to avoid them being used together so that it doesn't feel like you're losing a part of your damage when using these actions to adjust your rotation.

3. Adjust SMN's single target roation to combo your AoE spell off your single target spell in any example like how Shadowbringers Phoenix worked. AoE buttons that exist for AoE only bother me because we don't have enough engaging AoE situations to warrant needing a separate AoE rotation, and SMN dedicates a lot of hotbar real estate to buttons that almost never get used outside of dungeon trash. If we make all the AoE spells combo, it adds an additional modicum of zest to SMN's gameplay and makes it so that half of SMN's action list isn't just AoE trash burning.

4. Potentially have Leviathan, Ramuh, and Shiva upgrade your current summons outright and just give the player to choose between which summons they want for the animations. In other words, your summon phases all get a DPS boost, and you have the option to choose between two summons for each phase visually. The suggested solution in TC's post is also an option, but like I said before, there is the concern of the added punishment on KO if your stronger summon phases are locked behind each odd minute of gameplay and can really throw off your rotation.

5. Add Alexander as a demi summon that can be summoned in place of Bahamut or Phoenix and exists on a longer cooldown. Or in other words, you save it for buff windows and summon Bahamut and Phoenix outside of buff windows.

6. Replace the Energy Drain > Fester/Painflare system into something that uses one-off attacks from Bismark, Ravana, Susano-o, and Lakshmi. Specifically, I have the mindset that consuming a gemstone now activates a specific summon. For example maybe Leviathan or Garuda gives you access to 1 use of Bismark's attack normally, and then maybe you have a Carbuncle action that gives you all 3 of Bismark, Ravana, and Susano-o the replaces Energy Drain. And then perhaps Lakshmi is just a separate cooldown with a buff, like a physical Magick Barrier.