So I have restored vanilla settings in order to test the game as is with the new oval map layout

The layout is great but the AI still had significant issues with Marauders (large galaxy with all default settings + AI on Normal).
The issue is of two parts:
1. Marauders develop technologically way faster then enemy AI empires do and wipe their fleets even if outnumbered (which they are usually not)
2. The AI does not seem to "Pay" them to leave them alone, which puts the player into a significant advantage considering the ratio of payment to the costs of an effective defense fleet against them

These two reasons altogether lead to a situation where AI empires are crippled and the player can chose to bribe the Marauders thereby winning the game, or choosing to fight them and thus fighting the same faction in each game, that is not able to win the game as they will not venture out of their zone of control

Now I assume the tech advantage is there to give an incentive for the player to engage them in the hopes of capturing advanced tech but this also seems to be the case with pirates so my recommendation would the following:
1. Marauders should only be able to research tech that has already been researched by one of the in-game empires
2. They should never use battleships as considering their role and lore they are not fitting
3. AI should bribe them, but this should be visible in diplo screen and the amount that needs to be payed to them needs to increase over time and after each bribe. This would force the player to engage them once the price is too high or not worth it, while also enable AI empires to protect their colonies as long as they can afford it (Phidi lore advantage would shine here)
4. Marauders would still attack all the time as the price would go way up if everybody starts to bribe them, and some would quickly not be able to afford it

I have to agree that right now it seems that the AI really struggles with Marauders. Depending on where the Marauder worlds end up they can often completely box in and strangle certain AI players. I've even seen games were the Marauders take and hold planets from the AI very effectively. It seems the Marauders build too fast and tech up too quickly compared to the AI players. I think the escalating bribe cost idea is interesting, as it would make the decision of 'try to smash them now before they go too strong' or 'pay them off until I get stronger' more interesting.

-Will

Current version has improved a lot in this department. Marauder bases seem slightly less common, and their tech development seems to be in a suitable level now

If the AI starts paying them protection money to them, we are set

Another way of going about them would be to strengthen the AI to better muster forces for countering them. I have seen a few very good attempts but the AI attacks on these pirates systems mostly fail. This is not necessarily a bad thing (players sometimes also miscalculates their chances), but the AI should never sacrifice their forces if there is nothing left to protect the systems afterwards and hence it should be avoided that an empire has a fleet power of <10 ships in mid-game without waging war on any other empire - But like I said: it would make more sense (cost effective) to allow the AI to pay protection money and probably also easier to program