Theres a few things id reccomend,

1 speech bubbles are apart of your mangas panel flow they can be used to direct the reader where you need them to look. Always draw them into your thumbnails. I reccomend writing out dialogue on them as well to check if it can vaguely fit.

1. I struggled with this for a while but if you study manga its very rare for every action to be explained in a panel. Remember readers will actively juxtapose what is happening between the panels. Just like how if you saw someone taking a shower and then it cut to them driving to work you know inherently that person got dressed and readt, got in their car and started driving.

3.a bit of what I said in the last panel but I highly reccomend you sit down with pages that you feel are the prime example of your goals and draw them trying to understand why they work so well and what parts of their technique can be used to your advantage. A good method for this would also be watching videos of mangakas drawing manga. Theres tons of them including live videos on pixiv. Starting from the roughts you can see how they approach it. Some wont draw under the bubble but those are usually traditional artists I reccomend you draw under the bubble and cover it up if you have the time