It sounds like a decent way of doing it.
As a buyer of advertising services, I need your support in helping my stakeholders (approvers, internal customer, procurement, legal etc) understand why we should hire you.
Their workflow is usually based on MS Office + PDFs. I would go nuts if every agency came with their own customer portal that I (and everyone else) needs to logon to.
If you want to know what people value / read in your proposals, I would set up a call or better a in-person meeting with them, even if you lost.
Creative theft is of course always a risk, but I don’t see a technological solution for that.
Some of the best proposals are targeted, short presentations (live) that clients can ask customers. Send out a link for a zoom call using paid, targeted advertising to reach people, then get that focused number on a call. Nothing beats people investment imo.