Maybe you can let it electronic galvanizing? Hydrogen embrittlement is mostly only problem with thermal galvanizing. Also keep in mind the tensile strenght, this has impact on the fact you will get hydrogen embrittlement.
See also here for some more info: https://galvanizeit.org/knowledgebase/article/hydrogen-embrittlement
Looking pretty good!
Maybe another possibily to use in the future may be 2 or 3mm HDPE sheet. Seems to be a little bit more rigid then rubber belt, but still flexible etc.
Some nice first gear ratio. Overall I think it's better to have some more spread in high and low range for technical driving compared to stock gearing.