That's the point, it's meant to be niche. Like Feudalism. SD is not meant to be "worth switching" 99% of the time. It's niche is that of a gov for a civ that has no need for military and is looking for to win peacefully. In in that it already does insanely well. In real life, nations that are social democracies have very little military spending, huge welfare states and rely on powerful allies to protect them. They wouldn't survive on their own in a world with aggressive and powerful neighbors. You can't have both guns and butter.
If you're already on a war path then staying in Rep/Mon for the whole game is the way to go in stock game. Stock game's brutal anarchy makes having different govs later on pretty moot. You'd have to make them super overpowered to even make switching remotely worth it. Not even communism is worth the switch in stock game if you're religious. Scientist farms are too strong. You can just get 9 beakers for doing nothing.
Alright, well in that case, leave it as it is. However, do note that other AIs may also very well end up changing into them, which in most cases nerf them by quite a bit bc of the very high unit cost. For that reason, I'd personally recommend to lower the unit support cost for balancing as mentioned, but in the end it's your mod and your game.
I agree that the anarchy phase in the stock game is far too long. I've been working on a realistic rebalancing mod for many years, and I'm about 80%+ done, and one of the changes needed was bringing the max anarchy period back to 6 turns. I can't get rid of it entirely, first because I do think anarchy periods are realistic, but most importantly bc religious civs actually have 2 turns rather than 1 turns (like non-religious have on Sid difficulty). So I'd end up making one of the religious trait effects weaker than non-religious ones, which is something you don't want for balancing. Therefore, I think anarchy periods for non-religious civs are fine in between 3 and 6 turns.
However, I do strongly disagree that communism isn't worth the switch in the stock game, even if you're religious. Maybe that's the case on smaller maps (for several reasons), but not on larger ones. I was Republic before switching to Communism in my 40% standard map game. I was producing 1030 uncorrupted shields, generating (2907 gold from cities - 1368 corruption and 314 maintenance) = 1225 uncorrupted gold and 3042 total productivity (787 excess food). I had 356 specialists, which were all either engineers or scientists, which would equate to another 1068 beakers, or 712 uncorrupted shields. However, without use of exploits, engineer shields only count to city improvements, not units. Didn't quite name how many of both, but was rather equal. So that's another 356 uncorrupted shields and 534 beakers. So adding beakers to uncorrupted gold, that equals 1386 uncorrupted shields, 1759 uncorrupted gold and 787 excess food. I had no units over the unit support limit.
After I switched to Communism, few turns later I was producing 1952 uncorrupted shields (and still rising due to many factories I was still producing in many former non-core cities), generating (2030 gold from cities - 517 corruption and 457 maintenance) = 1056 uncorrupted gold and 3922 total productivity (964 excess food). I had 282 specialists, of which 167 policeman (which already count towards the stats found in the demographics), and the remaining 115 either engineers or scientists, which would equate to another 345 beakers, or 230 uncorrupted shields. They were about equal as well, so that's another 114 uncorrupted shields and 174 beakers. So that equals 2182 uncorrupted shields, 1230 uncorrupted gold and 964 excess food. Also no units over the unit support limit (duh).
In total, that's a
57%+ increase in uncorrupted shields, a
30% decrease in uncorrupted gold and
22% increase in food. With surplus gold, I was still teching the last Industrial Era tech (Motorized Transportation) in 5 turns from start to finish as Communism, so it was a very welcome change. But most importantly: first of all, my shield production was still drastically increasing, since now all my former non-core cities would soon be producing factories, increasing shields by far more than it already was. And second: uncorrupted gold is about 3.5 as less valuable as uncorrupted shields anyway. That's because wealth is an absolutely worthless thing to be producing at your cities (produce units and conquer enemies instead), and your gold should only go towards teching (which I had enough from), upgrading units (3 gold per shield), or hurry production (4 gold per shield). Also, despite me having a far tech lead, Communism has veteran spies as stealing tech as a back-up anyway, which is actually a more cost-effective method to teching yourself from the mid-Medieval Era. Using that as a base, we get the following stats:
Republic: 1386 + (1759 / 3.5) + 787 =
2675, increasing only marginally
Communism: 2182 + (1230 / 3.5) + 964 =
3497, and shields still rising significantly
So in conclusion, even with a
2-turn anarchy period as I had, you would get your 2 x 2675 = 5350 loss back within 3497 - 2675 = 822, so 5350 / 822 =
6.5 turns. I could lower it to 6 turns as my uncorrupted shields kept increasing significantly.