I simply cannot juggle defense and offence becauase as soon as my army is on one planet ready to attack, I have to move it in-order to deal with a distant uprising and by the time I move them all-back another attack is underway.[/b]
Defending solely with turrets should be quite possible until AIP gets fairly high if the difficulty isn't high. Say, 10 tractor turrets under a forcefield near the wormhole and a bit further back (in the direction of your command station) but still in-range have 20 basic turrets, 20 laser turrets, 20 mlrs turrets, 20 missile turrets. Having 5-10 sniper turrets scattered around the edge of the planet isn't bad either. More of each can be added, obviously, if there's a need.
Later on, like if you're getting mkII waves or mkIII roaming threat, etc, then you may need higher mark turrets and more concentration, but by then hopefully you've "spent" the AIP (i.e. chosen planets to take) such that you only have to defend 1-3 wormholes to hold the core of your territory. If you get to mkII waves and still have like 10 wormholes defend then you could get into a situation where you have to defend with your mobile fleet, yeah.
My AI is 371(II) but I regulary get attacked by lvl 3 ships. Is there a penalty for being slow-minded? (does progress/difficulty increase with time?)
There is a setting that increases AIP with time, not sure if you have that on. And as the AI continues to reinforce its planets over time it will eventually reach the limit of what can be on a planet and the excess can wind up coming to look for you, though iirc it's supposed to stuff those in barracks (which don't come looking for you) and then carriers (which do come looking for you, but you didn't mention any). Alternatively, if you've shot at ships on an AI planet then some or all of them (depending on how many distinct guard positions you shot at and how big your force was) will release from their posts to come hunting you. If you've got defenses they'll stop on the other side of the wormhole until they think they have enough free ships piled up there to take you, then bam. And those could be mkIII, or even IV or V if you hit a planet of that level.
I don't know what to do, this game has so much depth and I can't even get my toes wet, so-to-speak.
Any advice?
Start a new game against two diff 5 (or even diff 4) "Vanilla" AI-type AIs, making sure AIP-over-time is not on, and pick a map that's reasonably easy to find chokepoints on. The Concentric type is pretty good for that in my experience without being _too_ easy to chokepoint; the X type is great if you want something that's easier to chokepoint but isn't quite the totally linearness of Snake.
(naturally, Chris ninja'd me)