Well for me to tell you what you`d like I`d need to know your personal bio, but I don`t know you so all I can do is tell you what i like.

I`m a randomised man. I put all the options on random and take whatever the game throws at me (the only thing I choose is map size which is usually Large).

Strategy games always get boring in end games, it`s almost impossible to prevent. You get good, you get powerful, no one can stop you. I don`t even know how to stop my best friends from getting bored in late game, it`s all down to you and what you like. personally i like it when they get boring at the end cos it usually means I`ve gotten good and now things get easy. Just like in real life when I first started as an apprentice it was easy, now i`m master of my trade, it`s boring.

If real-life is like that, games aren`t gonna do much better, despite what strategy gaming devs say.

I totally relate early game is the best. Every action you take matters if you mess something up you'll lose momentum, which is interesting. But later you might find yourself wasting a lot of time by doing irrelevant things when you could just remove AI outright the moment you realize you're capable of and then quitting the game halfway through conquest because it's obvious who won.

I usually play everything set to occasional medium/large maps full of incredible AI.  My favorite play style is 1 planet challenge, I don't sweat colonizing, growing population, building improvements, dealing with upkeep of all other planets. I just take one from AI when I need more, which is usually already packed with stuff using godlike production. Basically playing as a looter, you should try it's quite fun.

I think having optimized custom race is too overpowered one knows exactly when and what to do. The more I play the more I prefer randomness for better experience. That's why I wish there was a 'random' option when choosing race for a new game, every time you'll have to adapt your game plan utilizing traits and abilities of that race. This and ascension victory condition would be a lot of fun in multiplayer especially. 

Well i dont like to play where you are handicapped i think the ai needs to play better at higher difficulty instead. I play everything on abundant except pirates that is rare on ludicrous with 46 opponents. The problem with a scientific victory it is a specific branch instead of have percent of all the techs first, or points. Number of points, or techs above everyone else after so many years, or weeks. I would like to see an adaptive ai.

Try playing a synthetic race. It's a very different situation.

also try mixing up the difficult levels per opponent. Put at least one on godlike, a few on genius, some on normal, and few on easy. That produces a natural imbalance in the galaxy that gives military fodder for the more adept civs. 

Large / Tight Cluster / Uncommon Stars makes a more interesting exploration phase and galactic topography.

The mods that provide extra planetary goodies and event are easy to add and spice up the game a bit.

And if you want something crazy, declare war on every race as soon as you meet them.