Ok, since that post I got a new job in programming, I think I might be able to pull this off with proper guidance. Would anyone like to cooperate by telling me which file I should be looking for to modify?

Thanks

Set it to diplomatic victory. Its like conquest, only you usually set the terms for when it is over. 

I rarely ever finished games or got close before doing this, which is why I think stardock never get large maps submitted , when it comes to the end game there needs to be more shakeups to keep it interesting, but I understand not all would enjoy this if CK2 rebellions and such like are anything to go by, very mixed reactions. Hopefully we'll get that as the game expands with events and such like that we can enable or disable.
 
Respawn Civilizations Yes/No
Breakaway Civilizations Yes/No
Political Rebellions Yes/No
Invaders Yes/No
Titan Monsters Yes/No 

You get the idea. 

Ok. Well you'll want to edit improvementsdef.xml and find the civilization capital improvement. Then give this a rather large bonus to planet resistance and defense. 

That will make capital worlds hard to take out. You might want a ridiculous bonus to keep things interesting. 

As to modifying game modes? No idea. once you do, you'll want to teach the AI how to play this scenario. Naselus would be a good guy to look up in regards to AI training. 

I thank you all for your answer. This community is really friendly, gotta love Stardock games for that! I have to admit, I completely forgot that changing the victory condition and defense bonus would not be enough, and that I would need to change the AI behaviour. That probably makes it a little bit out of reach for me.

I will fiddle with other victory conditions though, and will experiment with different map type. I'm thinking huge map with extremely rare habitable and extreme planets, maybe it will be less hard to conquer. Glad to see I'm not the only one who cannot finish games hahaha!