Is it a risk going bigger?

You did not post photos nor discuss height, weight, or measurements. But in general, with all that you have experienced, I would not recommend going with such a large and heavy implant that will thin your already scarred and stretched tissue even more. If you are doing well, just keep what you have and be happy. The 750 cc implants will be heavy on your neck and shoulders and will sag with time.

I am sorry to hear of the problems you had but it sounds like you are doing well now. You are very fortunate that you healed from your last implant insertion as it was only 2 months after you had them emergently removed. Going from 420 to 750 cc is a big increase in size. In my opinion, without knowing all of the details it is best that you not take any further chances and leave your breasts alone and do not consider implant exchange to a larger size.

Dear Friendly,
Going in for a fourth surgery to augment your breasts to a very large breast implant is not a good idea in the face of your prior history. Your risks for complication would be significantly higher for several reasons. Keep what you have and run with it. Good luck!

As a general statement, anytime, excessively large implants are selected the chance of undesirable side effects, complications and need for revision surgery will go up.
To make an assessment regarding the outcome of any plastic surgery procedure or the ability to make an assessment we need to see quality pictures, and or before and after pictures.
Considering your background, it may be wise to leave good enough alone.
Each time somebody has repeated surgery. The chance of long-term problems will also increase.
Follow up with your plastic surgeon to get a better assessment and better understanding of treatment options.
Implant Envy is a real thing.Best,
Mats Hagstrom MD