Welcome to the forum.
Just finished some brain surgery so I apologize for the slow response to your original post.

Garter snakes will eat all the rosies you can feed them.
Then they will develop a B1 deficiency and probably not make it.
Supplementing B1 will not stop this process.
Guppies are a much better live fish choice. Fresh salmon/trout/tilapia are also safe but you must be wary of the source.
I live in Iowa so getting "Fresh Fish" is just a marketing tool. I do not buy anything that has been processed or frozen.
Night crawlers and frozen/thawed mice are a good combination of food.
You are right about the varied/balanced diet, I think it is the best.
I will attach a link to the care sheet which may answer some other questions you may have.
Please feel free to post anything not covered and this wonderful forum and its members will be as helpful as possible.

Garter Snake Care Sheet - Caresheets

P.S. Marley is an awesome looking little scrub.