Now I have a little time to reply, I hope I might be forgiven that I drag up this topic again.
I think it bears remembering in the discussion above that random technologies are given as a solution to a deeper problem of optimal strategy being easily determined without it. It can also be the case that variance in starting position provide adequate differences, where yes, every player gets exactly the same options for technology, but the feasability of pursuing them varies wildly.
One way to go down that route would be if you’d introduce rare (or strategic) resources, which spawn so infrequently that each player only has access to a few of them within their realm, and then make certain research/development paths dependant on them. i.e. a special warp drive which needs a certain resource, and in ever greater abundance when you gear your development towards it. The downside of such an approach is obvious as well: your optimal strategy is determined by your environment, and generally easily deduced from it - do I have enough of rare resource X? It takes player agency away, much moreso than randomized tech. None the less, a pinch of such an approach would be worthwhile, if only for the resulting trade.
I’ve played a number of city builder type games in the past, and the basic problem there is that no matter the starting variation, there is a certain optimal ratio to be established, a certain blanket pattern that is 90% effective. There is only so much you can do with the environment.
Ultimately, you’re going to need to implement interaction with other players as a differentiator between the viability of strategies. This requires a very delicate systems of degrees of co-operation and hostility. A very detailed system of diplomatic treaties and trade might be a must on the co-operation side, having smaller increments of hostilities than invasion and raiding on the other side too, for that’d be an act of war already. But what abut spies stealing maps or tech, what about sponsoring privateers, siphoning off some tax revenue by flooding the market with drugs (opium wars anyone?). Smaller steps are needed.
Another thing, in designing the decision tree for the game, there needs to be more care that feasable alternatives are presented. The present optimal strategy is directly going for colonizers, more people more taxes, etc. it’s a snowball that wont stop untill corruption gets serious. So then, ask yourself as designer: what options can I present to be a feasable alternative for rushing colonizers? Invasion is too far of in the tech tree, and by the time you get there, you’re way too far behind on industrial base. Entertainment centers for more taxes is the next researched after colonization, so you can’t get an advantage there in time for it to pay off either. And so on. More care is needed here.