I believe it's a great idea to move to modern IDE. FlashDevelop was amazing, but it's too outdated now. Your work tools should be as convenient as possible.
VS Code is a great lightweight IDE, though it's still not as user friendly compared to JetBrains IDEs.
IDEA is amazing and very user friendly. Another reason to stick with IDEA is that if at some point you'll want to move to Unity, you will be able to quickly switch to Rider - clearly the best IDE for Unity and your habits of using IDEA will help you to start quickly and feel at home.
Though as IDEA Flash plugin is developed by JetBrains themselves, you can't be sure that new mobile development reality will be adapted in the IDEA UI in the future as Flash seems to become too niche to get their attention. From a flash perspective, it looks like VS Code is safer as a future-proof option.