as much as one wants, with all one's might, wholeheartedly
いつか心の準備ができたら—その時は本当に 思いっきり！
Someday, when I'm ready—then I'll really go all out!
— オールド・ボーイ (2003)
思いっきりやりましょう。
Let's give it everything we've got.
思いっきり押して
Push as hard as you can.
— TITANE/チタン (2021)
早く外に出て思いっきり遊びたい。
I want to get outside soon and have a blast.
泣きたいなら思いっきり泣けばいいじゃん。
If you want to cry, then just cry your heart out.