In theory you can apply for citizenship shortly at the 5 year mark. But of course it takes a while.
ARI has to be renewed at two year mark. I believe you then renew at the 5 year mark, or apply for citizenship, there isn’t a second renewal before the 5 year mark.
I think in practice the answer is that you don’t know the whole time line. That’s all a long ways off and a lot can change between now and then. You might not even be able to get citizenship by then - maybe your provider or lawyer promised you but the government sure didn’t promise it won’t change the citizenship requirements, and all ARI really promises you is residency. But ok, let’s assume that the path doesn’t change. SEF might drag feet forever, or maybe it all gets computerized and everything’s processed in 15 minutes.
Also, 1.5yr for ARI is is just the far end of the range of projections. It’s fairly clear that a determined someone who gets the lucky lawyer draw (someone aggressive on finding appointments) and is willing to sink the cash into being in-country for long periods or forking for last-minute flights can get processed a lot faster - but that’s just not feasible for any number of people. Being single vs a family of 5 is yet another monkey wrench - it’s so much harder to get large blocks of appointments, versus onesies-twosies - which adds a multiplier to that cash cost of last minute appointments or spending time in country. One person I know who submitted same time as I got pre-approval already, whereas me plus wife, we’re still waiting weeks later.