It seems a bit easy

[Suicide Junkie]

Play By Web is the way to get challenging opponents.

One that nobody seems to have mentioned yet is Pirates & Nomads mod.

Especially if you like fighting tactical combats, or focussing on a small group of named ships.

Play as a pirate or nomad against regular AIs, and you will almost certainly not win
Struggling to stay alive with a tiny fleet, building secret bases in nebulae and storms, raiding enemy ships for tech and resources.
You can do quite a lot with a single-digit number of ships

PS:
Most mods will require parts of the imagemod to be installed.
http://imagemodserver.mine.nu/other/MM/SE4/imagemod/
Download the imagemod packs, and install them over top of your stock pictures.
- They do not change any stock images, but simply extend the list of available images and animations available for use.
- Installing it to your stock folder means you only need one copy, and it will work for all mods instantly.