Honestly, I don't think that the solution you describe is a root cause fix. I see it as a band-aid. If the problem is growth, then it's grown that must be fixed. of course, you then do go on to talk about reduced growth. I think that the real solution needs to be something along the lines of no growth, or very low growth, perhaps on the order of 1% per year (or 0.1% per month).
Cralis has told me that one of the reasons that Marvin put in such high growth in SM2 was that players didn't like the nearly non-existent growth in ISF and supposedly wanted to see their colonies grow into large populations. But that explosive population growth produces explosive economic growth. Thus, I think that the game is better off accepting very low (or no) population growth as the price for maintaining a more reasonable level of economic expansion.
As for complexity and ease of play, I don't envision any of my potential ideas being any more complex than what exists now, and perhaps even less complex.
EDIT: One other option for tweaking growth that I probably wouldn't use, but is worth mentioning is requiring population growth to be based on PTU's rather than on PU's. I think that it's safe to say that in the PU/PTU model, PTU's are the real measure of population whereas PU's really are an economic measure. If growth were based on PTU's, players would see aggressive economic growth in colonial populations, and then start to see economic growth slowing when population are more mid-range (i.e. Small and Medium), and then growth much more slowly once populations become Large and Very Large.
But the cost of this model is that you'd have to convert each population's PU total to PTU's, apply growth, and reconvert back to PUs, which would be a serious pain in the arse for monthly growth and only slightly less so for yearly growth. But it would probably be one solution that would help curb explosive economic growth.
And it might also help reduce economic explosiveness to ban growth on non-habitable (i.e. Desolate and Extreme) worlds. And if in parallel with PTU based growth, this would ease the burden on the number of calculations needed during growth periods, if only habitable worlds could grow.
I took a look at PTU growth in a spread sheet. It's amazing how well basing growth on PTU, rather than PU, curbs explosive growth! As you move up from one bracket to the next, relative PU expansion rates decline, though as you grow WITHIN a given bracket, the relative PU growth rate increases until you flip to the next larger bracket. I attribute this to the PTU to PU conversion (the part of the conversion that factors in the PTU's of the next lowest bracket into the conversion process) adding a little wackiness into the overall mix.
Using the Ultra pop table and a 20% yearly
PTU growth rate, it took 11 years for a Medium to become Large, 15 years for a Large to become Very Large, and 12 years for a VLg to max out at 3000. (This is ignoring any floating PU caps, and no EL considerations.) I suspect that the reason for the Large's rather high time was due to its PTU conversion rate. I suspect that if its PTU conversion rate was tweaked a little, it'd be entirely possible to make it so that a Lg grew to a VLg in 11-12 years, same as Medium and VLg. Oh, and also, it took 8 year for a Small to grow to a Medium (assuming a minimum Small pop of 201 rather than 181). Again, I attribute this speed to the PTU conversion rate.
Of course, doing PTU based growth may be more of a hassle than some players are willing to put up with, if they're playing P&P. (With computer support, it's entirely invisible, of course.) However, I think that there's a way around it that's a bit less painful. It would involve having a separate PU Growth rate for each bracket that represented an average PU growth rate based on the actual PTU growth rate. That is, if one assumes a 20% yearly PTU growth rate (or about 2% monthly), for a Medium population that translated into average PU growth rate of about 6.55% yearly. Or 3.45% yearly for a VLg. Or 4.85% yearly for a Large. Or 12.3% yearly for a Small. (And 20% yearly for Settlements and below, since 1 PU = 1 PTU for those brackets.) Note that with this method, there is no built-in way to handle transitioning from one pop bracket to the next as is the case with strict PTU based growth. (And BTW, note that these are very rough numbers for the PU Growth rates.)
And another thing that could be used to limit growth on top of this is to get rid of free colonization PTU's. Force players to take PTU's out of their population to get new colonists. However, it's worth noting that this isn't as big a sacrifice as it may sound, particularly for a Very Large population. One might lose all the PTU's you need for colonists in the rounding during a PTU to PU reconversion after growth, given that each VLg PU = 500 PTU's in Ultra, or 450 PTU's in SM#2. Obviously, in populations smaller than VLg, with their lower conversion rates, rounding losses would produce fewer "excess" PTU's (i.e. due to FRD rounding).
Still, I think that isn't a bad thing. It would show that VLg pops can afford to give up population to colonization without any serious impact, whereas Large pops or smaller would feel an impact if they sent out colonists.