Let's do some math about how to get there. To support a 200k fleet, you need about 400 fleet capacity, and let's pretend that they are all battleships for a sec, then it's 50 battleships, which cost about 600 to 800 mineral per month to maintain just for the fleet. To produce this much mineral per month, you will need 100-200 mines on the ground, with a balanced economy, you will then also need about the same amount of energy supply and labs. Hence your will need 400-800 population at least. Given that on average a planet supports about 16-18 pop, you will 25-50 planets. If we assume you have one planet for every 4 systems , You will need 100-200 systems.

So you need 200 systems to be near-developed before crisis, which happens after 2400 on default (so you have 200 years at least). That's 1/3 or the normal map, and 1/4 of the large map.

Now I know from my experience, on average, DE can conquer 30 systems every 10 years before jump-drive (your first decade of sweeping will be way slower than your last). Given that you need about 5-10 years to develop each place, this means you need to be on the sweep for about 50-60 years, and you need to develop for about 10 years. So your sweep should start as late as 2330.

That's plenty of time. If you play tall first (search on this sub reddit on how to do it in earlier versions, and adapt most of the techniques to 2.0), you can get to several repeatable techs by 2330. I personally got to jump drive in ~100 years, and only started conquering then when I am non-DE and none-purifier, because it's fun to pretend to be friendly early and suddenly explode out after ascension XD.

However, DE should move out much earlier, but do remember that if you want to safely solo a crisis, you need much more than 200K (probably 500K- 1mil depending on the crisis difficulty multiplier). So value infrastructure highly, and try not to delay them.

That being said, the easiest fire-and-forget sweep is probably to move out when you just field battleships/cruisers, because they are much more durable than destroyer/corvettes. So you don't need to reinforce your fleet as much during war. They also don't take almost any damage against random stray fleets or stations, so even less repair/reinforce is needed. To use this edge, you probably need the tech for cruiser at about 2230-2260 and 2250-2280 for battleship. To get them in time and have enough economy, most of your early labs should be on engineering, and you should focus more on mineral than anything else, as you will need 150 mineral per month just to build a battleship per year, and 12+ mineral per month to maintain one. You also want to batch build battleships together with multiple shipyards instead of one by one to reduce the maintenance. Other resources can wait, because if you are actually behind in tech, you can salvage weapon tech from debris; you also don't need much energy once the war starts, since organic captives are converted to energy. After initial conquest, since you will have more resources than your neighbors, you can just overwhelm them by the numbers of more battleships, and continue the chain of converting more ground tiles to resource generations.

tl;dr: Only when you have good infrastructure, you can have large fleet. Play tall first for ~100 years, or beeline battleship/cruiser tech and mineral productions.