The Zealots faction trait is supposed to award you 15 points in your primary ideology per planet conquered. If that works (I've never tried it, though I don't recall seeing any bug reports about it), then that could help. You might even be able to abuse the ability a bit by conquering a planet, allowing it to fall to someone else, conquering it again, and so on, as the Zealots ability has a trigger of OnConquerPlanet instead of OnConquerPlanetFirstTime (which is used by the Malevolent perk that grants +1 morale per planet conquered).
You could also turn up the event frequency, if you haven't already. Or you could increase the habitable planet frequency and reduce the number of players, which means that you should usually have a greater number of worlds to colonize if you keep the same map size (there may also be some effects on the number of colonizable planets by map type, though I am not certain).
Let's say you start counting that from turn 80, which is perhaps a best-case scenario for colonizing 50 planets on a typical map
If you really want the full benefit of the ideological buildings, you really ought to unlock and build them as early as possible. Let's say that I can build the repeatable ideology buildings starting at turn 10 and that I can colonize 1 world every 5 turns starting at turn 20. If I rush the ideology point improvement out first thing (e.g. with the Colonizers faction trait so I don't need to waste income), then I will have 69*0.1*2 + 59*0.1 + 54*0.1 + ... + 4*0.1 = 51.6 ideology points from improvements by turn 80 just with 13 colonies and the homeworld. If you assume a higher growth rate (e.g. your 50 colonies by turn 80 objective, which could be ~two colonies every three turns starting on turn 15), you could get a lot more than that. Two colonies every 3 turns starting on turn 15, with repeatable ideological buildings unlocked on turn 10 and built as soon as possible on each colony, gets you 156.8 ideology points from improvements by turn 80, which is enough to shave ~30 turns off your estimated game duration.
It may also be worth taking a short dive into Benevolent for the free colony ship and into Pragmatic for the three free construction ships (which you can upgrade to colony ships and then use without costing homeworld population) even on the runs where you're not trying for the Benevolent or Pragmatic achievements.
Aside from that, if you were not opposed to modding the game to do this, then it'd be easy enough to increase the rate at which ideology points are gained, or perhaps reduce the cost of ideology tree unlocks. I personally feel that modding a game for the purpose of getting achievements is a form of cheating, however, as in such a case the goal of modding the game isn't to make the game more enjoyable for you or for other players. Unless you're involved in a competition with other players to get the achievements, however, it is not a form of cheating that I would consider to be strictly wrong, as you're simply reducing the amount of tedium you need to sit through in order to achieve something that you could have gotten but in a more boring manner had you not modded the game.
