To be honest, I still say you should get entrechment even if the AI suck with SBs. If nothing else just dont build them. The other updates to the other defesive structures should still allow the AI to give you hell if your not careful.
Besides Unfair Reseracher usually does well with is SB upgrades.
1) I'd concur with this assessment. Not building star bases is the easiest fix, until (if) the dev's do something about it. Additional tech upgrades your hanger bays, weapon platforms, and inhibitors, with entrenchment, and I've noticed the AI makes relatively good use of that research (though application through building those logistics can be a little lackluster).
Know your difficulites; easy, med, hard, unfair; defense, offense, researching, resources. Respectively.
2) I too, am currently only on Singleplayer play, no MP (yet). However mods (as said above) are the most important factor, if you're into trying out some mods. We seemingly bought the game within days of each other going by Member IDs, and with sins 1.17, virtually no mods are compatible, and most modders state they won't go past earlier versions for sins alone. Granted, I am running into the same issue with entrenchment, 1.03 is not compatible with most mods (yet), or they sort of are, but minidumps plague them. The difference is, that's a work in progress, not a dead end like in sins alone.
I would suggest avoiding mods that allow you to ramp up your technological edge, as this can easily outweigh whatever tactics you apply to "help" the AI in the beginning.
3) I guess your play style might be the most important factor. If you like epic 40 hour long games, (ancient gifts, huge multi, or aerolian sector for example) it's pretty good. If you like quick 1 hour maps or games, then yeah, maybe entrenchment isn't for you.
With the longer games, you just have to allow the AI opportunity to amass resources / tech. Akin to giving them a head start, where I've literally only built up enough to defend against pirate raids , but otherwise, doing nothing with only one planet for the first hour. Try all allies to begin a game, by the time they declare war on you ignoring them repeatedly, they have that head start. (its's actually good MP training too, but you're not into that, so I digress.) With entrenchment and the unfair difficulty setting, it's all the more impacting. Pick one AI enemy, and just use your money to more or less make it so the pirates only attack that one faction, so you don't dillute the other AI's head start. This way once you're ready to "begin" the game, you still have low overall resources. It's a moot point overall if you have 20k of each once you "begin". Lastly, give your resources you don't otherwise use to your enemies.
4) Enchtrenchment is an expansion, not a microtransaction pack, or an update. You can actually run both Sins and Entrenchment at the same time..On hard, there is a difference, but it's not game breaking, and the new techs / units help narrow that margin. With unfair difficulty, again, all the more.
5) Lastly, remember the threads you're reading are from players who do dabble in multiplayer, to some degree. Their level of difficulty that they are used to (with human enemies) really turns down the notch for them on AI difficulty even on unfair. But for you and I? Not so much.
For example I can't really play Medieval: Total War 2 anymore because I wasn't good enough to really compete with those guys in MP, despite trying for over a year, but the experience made my singleplayer games too easy.
Turning away such a chunk of content (including mods) despite one unit you can opt not to build...I just want to make sure your decision is based on more applicable information as far as you're concerned.