The problem was never that siege frigs were too strong. They were not. They sucked all along with the way too high 12 supply.
The only problem was that the AI would build them despite then having no more room for a real fleet and then losing miserably.

Well, it breaks a few of other things badly too:

1) due to the XP from destroying siege frigates being 3x more than light frigates for far less effort, making loads of capital ships is by far the best strategy in SP. You can get by just fine with repair yards and cap ships until you hit support cruisers. Removes all but 2-3 cruiser types as necessary or even useful in SP.

2) the huge fleet cap usage from a fleet full of SFs means the AI needs to put its fleet cap up far more than any sensible player, which combined with its miserable spending on infrastructure, trade, research etc just cripples its economy. So the next kludge we end up with is an 'unfair' AI which just ignores the economic game entirely. Oh boy is that fun to fight against, it's like fighting the king of idiot rushers being fed by someone elses economy.

3) due to the siege-frig / run away idiocy, we end up with a massively turtle friendly PJI, crippling the efficacy of raiding and reducing the incentive to take a cap ship just about anywhere hostile. Game breaker.

So that's .. 3 kludges, all of which make the game actively less fun, because nobody can f*cking sort out the build priorities for the AI.

Ironclad, do you think you could make the AI external to the executable and let modders play with it - as in, next patch? Please don't make the patching game a kludge to fix a kludge to fix a kludge to fix a kludge. The approach you are taking right now is making the game continuously and cumulatively worse.

I play against the AI regularly and while they may send decent size groups of Siege frigates agaisnt me, I don't findthem to bethat much of a problem so long as I keep good defense/offense near my front line (as anyone in a war should). I think they're more of a problem if you turtle a lot, but if you have at least some offense going on, then they're not really a huge issue.

Now that we have a -700% PJI it's even easier to get by with just cap ships and support structures. Don't even bother researching ship types until you get to robotics/overseers/guardians which you use to buff the caps. Just make hangars and repair yards and turrets. You don't need that many fighters to hold off bombers, and the repair yards can keep a cap ship alive practically forever. I don't think I have ever lost a cap ship to the AI in friendly territory.

Actually, the problem was although Siege were easy enough to kill, it took more effort to kill them than it did to build another wave.

Personally I have no problem with reducing their survivablilty even more with added bombing capabilities. You can't expect them to come in bombing and surviving until you clear out all immediate threats but once that happens they paste the planet much faster.

WTF is wrong with you siege whiners? Have you not discovered the wonder that is 2 turrets or 2 hangars in defense? I mean jeez. The AI is gimp now because siege frigates eat 15 supply and die like guppies next to sharks. FIGHTERS >> SIEGE. The siege frigate is now NEVER worth building compared to just paying for a siege capital ship.

Depends on the capital ship. The one on the right is the siege capital for each raze. Those work better then siege frigs, the others don't. I mostly play Vasari but their siege cap can spawn little siege cannons that on their own are about as good as a siege frig and it can have 3 of them active all the time. That combined with its own superior firepower ... no way a siege frig fleet of the same supply can even come close to that.

If 2 siege frigs die attacking an asteroid, the cost for just re-colonizing it and building that pop upgrade again and are lower then the cost for the siege frigs. Not to mention the wasted supply the siege frigs cost which is missing in the fighting fleet then.
If 10 siege frigs die (and with 3-4 hanger defenses they will!) even losing a fully upgrades planet is cheaper then rebuilding those frigs.

Well, what they were useful for online was to break stalemates and to keep the front moving - once someone lost control of the space around a planet and if they'd had their fleet mauled defending it, a few extra siege frigates pounding it meant that they would not have time to build another fleet before you could take the planet. However, now that SFs are practically taken out of the equation, you might as well skip trying to take planets altogether until you've nursed a marza to l3 Raze Planet or jarassul or revelation to level 6. Without the ability to quickly reduce a planets health, border-area planets become something like a WW1 trench.

Someone who is any good at all now has enough time to come straight back in your face with LRMs and fighters and bombers and make it extremely difficult for you to use siege frigates and cap ships. If someone has 5-6 carriers with fighters they'll take every SF you've built before they can jump out, then use an LRM blob to force your cap ships out of siege mode. In the time it takes for you to deal with that, the planet regenerates another 1000-2000 health and the grind starts again.

Sins really did not need to be made any slower or any more of a grind-fest.