I believe there's improvements coming for forms however currently I have faced similar issues. You can extra the forms package to the /Views/Shared/ElementBlocks location and then customize them as much as you want. So at current the best bet is to do that and enhance the forms with any extra accessiblity features as required.
We are using the accessibility tool SiteImprover for an external site where we got an issue with some accessibility regarding Episerver Forms.
We are using Episerver Forms v.4.29.3 and when editors set the checkbox and radio input fields to be mandatory from the Episerver interface, the aria-required attribute is set on the div of the container holder for the elements. This is pointed out as an accessibility issue in the SiteImprover tool.
aria-required attribute is placed at the div when editor mark a checkbox or radio input type as a mandatory field in Episerver Forms.
aria-required attribute should be placed at the input element when editor mark a checkbox or radio input type as a mandatory field in Episerver Forms.