Applying to a second MSci/PhD with a PhD
If you got the maximum grade on a dissertation in another PhD program, that would be viewed positively. The admissions committee would be hoping that you will do the same in their program.

The basis for concern is that your pre-dissertation work will "average down" the value of your application. It's not something to be ignored, but most admissions committees will give the greatest weight to 1) the highest level of work that you have achieved and/or 2) the most recent level you have achieved. in both cases, your PhD dissertation grade is a big help.

I might refrain from taking math courses until getting into a program, even at the expense of being less prepared. Because if you take those courses and do badly, that becomes the new "most recent."