Its actually possible to get even more in lvl 5, but its a bit trickier. You dash jump from the third raised section at the start of the level, you can take a high road with some silver medels that ends right at the gold medel path.

best way i found to get points, is simulation lvl 5 get all gold medals, then before the operator is done speaking, rush to your left and get the silver medals. 480,000 each time with lightweight mode.

Once you get to 30,000,000 and get the anti ship energy cannon, then it's just all over. kill the last boss on lightweight mode in one hit

I found a good way to grind points in the final boss in Mission Mode with the lightweight armor equipped. It can hard to beat it since you take much more damage, but you also get double points, meaning you'll get 600k points instead of 300k. After several rounds with the final boss, you should be able to unlock the Anti-Ship Energy Cannon, which can destroy the final boss in a heartbeat.

Not sure, maybe the best is to grind a level that you are comfortable with and get the time bonus etc.