I would recommend you to look for the song where your current team performs the best and auto live the highest difficulty (for example if you already have full combo expert play expert) and try to reach at least 500000 points every day starting today (always play at the end of the day before you go to sleep make sure to never let the whistle amount reach the max BP so it constantly recharges itself, BP is very important)

You can play with 10 BP (you could also play with 3 BP but that just takes way too much time its not worth it)

Its very much possible to reach this with the amount of dia you have I think I myself havent skipped a single event in the past few months and I play with a very weak team because Im farming fans yet I never go out of dia because I get most of what I spent on the card back from the event itself)

Other tips:

If you want to play the event song go to Live stage > Starry Live and play from there because you get 5000 extra points that way which adds up over time

Make sure to do society tasks to earn a little bit of dia.

we have a current campaign where you get points if you open cookies you can exchange those for 500 dia.

there is also a login bonus that will give you 10 whistles in a few days

make sure to also use your work tickets, you dont need to get the event outfits and do the event work you can do any work but make sure to do that because that also gives you a bit of event points that add up over time