to add onto the above solution, you don't actually have to be strong enough to defeat them. you just have to be able to game the enemy AI. what happened in my game (before I fucked it up) was that the holy guardians spawned so I had the worlds, but the benevolent interventionist FE was the one that awakened, and I was more or less forced into becoming their signatory. awakened ascendancies are nearly impossible to catch up to in fleet power. if you don't wipe them out before they start cranking out massive fleets, you will never beat them in a straight up fight.
LUCKILY... the AI is pretty stupid. I had to win a war of independence against them to be able to declare war against the holy guardians on my own, but my fleet power was 200k and the AA was growing theirs at twice the speed. so what you do is build a bunch of corvettes (might only need one, but you want to make sure you have enough to keep their fleet distracted) and park them on the edge of your enemy's border. make sure their fleet stance is set to evasive so they hightail it out of there when the enemy fleet shows up, and also make sure each corvette is in a fleet by itself. take your main fleet and all your assault armies and park them on the border too. when everything is set up, declare war on the FE/AA/whoever else has the planets with only the condition that they cede the holy worlds to you. by this time you should have all the "cheaper warscore demands" techs researched, so when I did it the two planets only added up to 17 warscore. because you will likely be underpowered relative to them you will still need to invade several planets to get them to surrender, but it's just time consuming more than anything else. my main fleet never got touched by the AA's because they kept chasing one corvette in and out of a system right at their borders. send your corvettes to all the empty systems within their empire that you can find, and if there's one system that's decently far away from the planets you're looking to cap and is also just a single hyperlane jump away from the border that is probably your best bet. remember to keep an eye on your corvettes and if they start to get chewed up before they can evade the AA's fleet, start building some more (if you have a rally point set to your main fleet, you will have to manually select the corvettes after they're built and send them to another location to keep them from accidentally joining while you're not paying attention). with your advanced warp drive tech and strategic placement of your ships though you shouldn't have to worry about that - i.e. try to have them enter a system from the opposite direction that the AA's fleet is entering so they can't engage you right away while your corvette primes for the FTL jump
I hope that makes sense, it might seem a little cheap and honestly you can use it to win any war in the game (except endgame crises) which might discourage some people but given how nearly impossible it is to defeat an AA straight up I don't feel bad about exploiting the dumb AI. happy hunting!