I'd tread slowly with low-carb first to see how you respond prior to attempting a multi-day fast. Not so much as a preparatory step (although it's great for that too), but just to make sure you're not overly fatigued or impaired from some of the initial side-effect symptoms, which may impact your desire or ability to fast (especially if you've got work or other obligations to attend to). In theory the hormone hit that you're getting should help with some of the typical lethargy that can come in the initial days of multi-day fasting, since it will likely prevent your body from scaling back metabolism as much initially.
There's obviously the open question of if you'll even have a desire to engage in a multi-day fast at that point. It's not necessary for weight loss or a check on weight gain, although some enjoy other aspects of fasting or are chasing potential health benefits (e.g. autophagy). I'm honestly not sure if that's still present in the same way when on a GLP or not, but will be interested in hearing your experience.
As a data point, several years ago before I was really immersed in any of this stuff I tried a single shot of Mounjaro 2.5mg on a whim (long story) and went on to kick off a 120 hour fast. Was only able to get my blood sugar down into the 60s and I forget how high I was able to get my BHB (ketones) up to. It was surprisingly trivial to achieve, although instead of getting over the hump at that point, since it was the one and only dose I took, I felt a stronger desire to conclude the fast than usual as the concentration in my blood diminished.
DISCLAIMER: If you don't have experience with multi-day fasts, it's probably not advised to attempt one while on a GLP. It's a bit of a niche practice and there's definitely a learning curve involved.
