When should I apply for the DVT visa if I plan to return to Thailand in April 2025?

You could get everything ready (make a profile on the eVisa system, pick the consulate, visa, upload your documents) because you can store an application that isn't submitted.

Then as soon as you get IN your home country you could apply for the visa online. That would be the fastest you could do it

And that's not true you have to enter the country within 3 months of the visa being approved,. SOME consulates won't accept the application if you're going to thailand more than 3 months out,

BUT

there is no must enter within 3 months rule, the visa is good for 5 years <- meaning any time you enter during that time you get stamped in for 180 days

Typically 3 weeks would be plenty of time for approval. Stating what the home country you're applying from would be useful. Check the Thai embassy website in your country to find out their processing time.

I am.