1. Do an initial internal audit against a (few) known security standard(s) that are relevant to your org (ISO27001, NIST, PCI etc).
2. Create a System Security Plan document that outlines what your intended security posture is for each control in the standard you are following and also recording what your current posture is. Record evidence in this document too (screenshots etc) along with an caveats and mitigations.
3. Once you have the results create a Security Risk Management Plan (SRMP) with a risk register annex for an itemised record of gaps. This document should outline what gaps you have and categorise according to the the risk they represent for your company using the risk matrix https://en.wikipedia.org/wiki/Risk_matrix
4. Once you have the SRMP you can create a roadmap starting with the highest risk items and working down. It might take years for you to get to a good point but at least this process will focus efforts and give you a plan to tackle what seems like an insurmountable volume of work.
If you read this comment and thought "We should do this" I highly recommend to contract in a security consultancy to act as your organisations CISO to help you with this process. They will provide experience, structure and knowledge that is no doubt missing from your org. Rating risks can be non-intuitive for example.
Given that NIST is tracking so many controls (1190 controls & enhancements in the current revision) it becomes hard to see the singular burning trees from the forest view that is NIST.
I thought security checklists were long until I ran into ISO.
Edit: Typo (and coffee).
Shipping is easy. Shipping makes the product better. Shipping gets the customers and the funding and furthers the mission. You do what it takes to ship, to get the thing working, to do what you're burning to do, and the rest is details.
Taking the time to ensure you've thought through your authentication strategy and done your key management well does not directly do any of these. It feels like a waste of time by comparison. What does it matter? You'll fix it later. You don't need bullshit corporate malware endpoint management, you only hire good professionals.
-----
It's so easy to slip into that mindset. Because you mean well, you want only the best, but you have priorities to balance. I can empathize with everyone involved in a list like this... but speaking as a security professional I ultimately prioritize the safety of the user.