This implementation is unnecessary complicated. For the step update, you can use Out Of Band update: https://htmx.org/attributes/hx-swap-oob/ which works in a way that you can send multiple HTML fragments which can be anywhere on the page and HTML swaps them out. Good for notifications, step update, breadcrumb update, menu highlight, etc...

I usually solve the second problem by simply saving the state of the individual input fields, you only need a user session. Depending your use-case, you might need to be transactional, but you can still do this saving everything as "partial" and close the "transaction" (whatever it might mean in the given context) at the last step. Much-much simpler than sending form data over and over.