I'm going to say this under the assumption you're thinking of the OAS - the Titan - and not the ORS - the regular capital ship that can absolutely can't tank an SMAC and fleet support, barring some sort of hilariously corrupt download.
Truth be told, I'm pretty certain the only reason I can hold my own against the AI is because I've been playing the mod for the past several versions. I'd long ago nailed down my aggressive expansion technique. From the official explanation we've been given, the Sins AI can't handle the faster pace of the mod compared to the original game, and requires a massive increase in income to get it to achieve any semblance of working properly. I seem to remember some sort of upcoming fix with regards to this, but I can't quite recall and it's neither here nor there regardless.
I'm no stranger to RTS games
Then you should be perfectly capable of learning how to overcome it. It's a fairly formidable barrier to entry, to be sure, but it's very possible. Your optimal expansion strategies, which researches you need to get when, and how to squeeze the most out of a planet as quickly as possible. Maintaining varied fleets and keeping them mobile is very important, as SMACs can't be trusted to defend planets on their own with this release, and neither can you rely on frigate spam. You're on the right track building the capitals, but they need accompanying fleets to take full advantage of the utility of their abilities.
The OAS is just a hilarious fleet killer. You basically just have to completely overwhelm it. The most reliable way I've found to deal with it at any point in a match is to lure it into range of 6+ SMACs with a supporting fleet, but not too closely so that it can hit the stations with its multiple weapons batteries. Another way is to rush Infinity, since it can beat the OAS one-on-one.
Weren't post war vessels supposed to have shields? I know that the Infinity and Strident class frigates had them, and supposedly energy shielding was standard issue for ships of the line by that point, Autumn class included.
They did indeed, and shield refits for UNSC capital ships were even a part of the release before this one. However, they were removed for a variety of inelegant reasons. It was a hassle to go out of your way to drop it on existing capital ships after completing the research, the AI apparently didn't make much use of it at all, and (if you would believe it) they made late-game UNSC fleets nigh unkillable.
I will say, though, that the AI does seem to cheat. On more than one occasion I've brought down the OAS, only to have the game announce, "Enemy titan detected" >literally< within the next 5-10 seconds.
All in all, I'd recommend playing with other people over the AI. Better for practice, and more realistic.