On large maps against AIs, a foolproof strategy (for me) in Entrenchment / Diplomacy, for TEC, goes something like this:
1. Build an Akkan Battlecruiser, a few Cobalt Light Frigates, and use this main fleet to start taking over colonies.
2. Build 3 civilian research stations and 3 military research stations.
3. Rush to the Raloz Heavy Constructor tech, and use Raloz cruisers to get a starbase in every sector you've colonized so far. At the same time, research Trade Port technology, and the tech that enables starbases to have trade ports.
4. "Turtle up": this means stop expanding after a certain point, and just continue to upgrade starbases. First I upgrade their merchant docks to maximum; then I upgrade their defense and weapons.
5. Throw in a few Hangar Defenses in close proximity to the starbase, and research the tech (in the Security tree) that lets hangar defense shoot at fighters. Make sure to spend all the command points (for fighters/bombers) on all the hangars.
6. Throw in a Repair Bay or two in close proximity to the starbase.
The AI (at least in Entrenchment) is too dumb to effectively attack this setup until very, very late game. Sometimes they aren't smart enough to get through it at all; I have set my game to 8x speed and went to sleep, woke up 8 hours later (8x8 = 64 game hours later!) with millions of resources and the AI hasn't destroyed any of my starbases.
Once you have this setup, you can basically expand at your own pace. Start rounding out your economy, researching higher techs, building culture, and creating a large attack fleet. Then find the nearest enemy planet and start conquering.
This strategy is only effective if you are able to pull it off fast enough. The critical step, for me, is to get that first starbase built on my most vulnerable planet, as quickly as possible, and then get its hitpoints/shields increased with upgrades. Once that is accomplished, you have a lot of time to build up further, because the AI will throw small to medium numbers of ships at that starbase and they'll just get shot down over and over. You don't even need to station any ships in the gravity well, usually.
Watch out for the rare chance that the AI will deploy massive amounts of anti-station ships (e.g. the TEC torpedo cruisers). This usually doesn't happen until your starbases are already very fortified, and you can either micromanage lots of bombers to take out the anti-station ships, or fully upgrade your starbase's weapon systems so they can fire missiles at the enemy anti-station ships.
This strategy, being a turtle strategy, means that you'll end up having a very long game, and the first half (or so) of your time will be spent defending your territory from waves of enemies. Thanks to the massive power of the TEC starbases, this defense should be feasible. Once you are successfully defending from enemy waves, it's time to flesh out your economy, followed by your fleet. Then you can swoop in for the kill in one of two ways: you can either roll your primary fleet over the enemy worlds one by one until you win, or you can take a world, fortify it all the way up (upgraded starbase etc). The latter method takes much more time, but the chance of an AI comeback decreases with each world you fortify up. The larger the map, the greater the chance you'll need to employ the slow method to prevent the AI from recapturing worlds you've sieged.
