Kenstar, here's my take on the issues.
"All of the AI declares war on you" is a sign of your success.
I now plan on this event to occur.
What's the key to victory in such a situation? In Dark Avatar, it is military starbases. However, these have been moved to a very high technological expense in Arnor and therefore the starbase strategy is basically (unfortunately) removed from the game (anyone else have this feeling?). What to do in its place? Well, the defense technologies are much cheaper than the offense technology. Take a look at your biggest and baddest expected enemy. They have lasers? Make yourself shields, barriers, force fields. Up to about force field 1 is very doable. Imagine your 20 hp ship with 10 attack and 30 defense. I have decided defense technologies are the key to offensive victory for a number of reasons, including low cost of technology and low loss of ships. Just for calibration, I always play on IMMENSE maps with ULTRASLOW technology.
Furthermore, since you know the AI is going to declare war on you, sensors IV with Eyes of the Universe is an absolute must. This keeps the AI from sneaking up on you and you can deploy your ships most effectively to obliterate any AI that dares to come near you. Good news is that the AI doesn't seem to know that it's best strategy for conquering you is to prevent you from gettings Eyes of the Universe by building it itself.
I must say that I like Arnor but I'm disappointed that hte only reasonable strategy to victory is CONQUER & KILL. Espionage is semi-broken as pointed out in other posts (i.e., tons of money and very little chance to steal technology); it is excellent though for placing on farms before you invade; if espionage were working better, you could imagine overwhelming your enemy, stealing all tech, both based on a strong economy. Influence (even with Krynn) almost works; I have conquered a number of planets with constructor-influence attacks; but eventually AI decides to declare war on me and I don't have a strong enough military. Acension is one strategy I have not tried, just for personal taste (not too interesting to me). Technological victory is another I have never tried simply for personal interest.
Good gaming!