i think the answer to all your concerns is simply choice. currently every dps job in the game has you on a set rotation and combo that is optimal, and there's very little reason to stray from that other then its time to aoe vs its time to focus a monster.
There is no basic rotation exactly, there are no builder spells, your damage spells also build the bars, its just whichever one you choose to build up at that time.
For example, your starting a fight, you got 6 spells to choose from, cast maybe fire, your fire bar goes up by 5 (arbitrary number) after that stone and thunder proc. If you use one of those 2 those elemental bars will go up by 10 (again arbitrary). say you went from fire to thunder, then ice and water proc, you use water then aero and fire proc allowing you to go back to fire or go to aero. But every spell will branch off differently and cycle back around within 3 spells since they all proc 2. this allows you to focus around 3 spells if you want but otherwise you can also choose to juggle between 4,5 or all of them.
While your leveling up you dont need to have all of them in order to make use of a summon, you simply use the spells that lead up to the only summons you have. if you have only fire and air spells then you alternate between those 2 or focus on one and eventually summon ifrit or garuda. as you level up and earn procs as you learn more summons the job becomes more filled out just like any other job but your never unable to summon because you lack the other elements. Also currently the idea is that you have 1 elemental spell for each, but that would include fire 1, fire 2, fire 3 that all just get upgraded as you level up just like they currently do.
Also i think i already gave a number of examples of why you would use all the summons instead of sticking to 1. aside from the incentives it would be that you have to because the summons would be on cool down after you use them. The highest dps would be bahamut by filling all the bars evenly and to 100% but that would naturally take much longer then building 1-3 bars and alternating between those summons so its probably not the best thing to focus solely on. Not to mention its cool down would be much longer then the other summons.
I think part of the charm is that there is no set rotation, it wouldnt be something you learn and then mindlessly execute in every fight. You would make use of the summons buffs or attack benefits based on each fight. In theory it is very similar to rdm in that you build elemental bars in the same way and casting elemental spells proc other spells but its not so limited that you just cast A then cast B then cast A then cast B then eventually activate extra attack mode. In this way its more like, build whichever element you want and either summon immediately or save the summon for when you need it based on its buff. Like you can have the titan bar filled, the water bar filled, the thunder bar at half, and the ice bar at half, but you dont want to summon titan yet because your waiting for a tank buster to make use of his aoe shield (stone skin?) so you summon leviathan instead, when hes done that bar goes to 0 and you build a bit of thunder to summon ramuh, do damage with him then summon something else, saving titan for when you need him. or if your mages need mp and you use shiva. And when you summon its like changing your form because you dont cast your normal spells anymore, you solely control the summon and all your actions get replaced with his as a pet hotbar
i dont really know what to say about the size of the primals, i think thats also part of the appeal that they are not little things like they currently are.