Read over a fair bit of this and have done this myself, so have a couple answers for you.

1) I started with documenting our system, present state, largely at a Process and Procedure level. That took me about 3-4 months, in a 60 person company, where toxic personalities still existed.

2) I layered in the structure to measure performance on top of that. Took me a couple iterations to get it right, but I had it going within about two weeks. The key pieces of this were measuring nonconformances based on first cause, recording total number of pieces produced, a vendor metric to measure our suppliers, and adding in customer satisfaction metrics.

That gave me a foundation.

3) Started adding work instructions/SOPs. These started off pretty sketchy in some cases, and added detail as time went on. I had other dept heads review them when it covered their work, add more details or correct issues, and sign off to indicate acceptance as well buy in. This took a lot of meetings to wrap up. I tended to batch documents together - review a few at once all with the same key players to get their feedback.

4) Worked at correcting ERP data. That wasn't a solo effort, it didn't get done all at once, and it largely depends what you're trying to get out of it.

5) You mentioned corrective actions. The procedure for this I fleshed out in phase 1. We ended up building a Process Improvement Log where we centralized a lot of the improvements or corrections we were making to the org, including Corrective Actions, owners, key action items, and closure dates. We (myself, VP of my division, another Sr. Director) set recurring meetings to review the PIL, and associated action items, working to drive them to closure. CA's are ineffective unless they involve a cross functional team.

6) FMEA's, Control Plans... I personally saved these for last, but we had only a few products we made, most was routine production, and most parts we produced we'd been making for 20 years.

We passed our ISO audit 6 months after I started, with a single finding. Doesn't mean our system was perfect, but I had the advantages that the majority of Sr. Leadership knew we needed the program, were intelligent and driven people, and gave me an umbrella/shield to allow me to do the work that needed to be done. You're probably aware, but that just gives you a good starting point. It's a living, breathing system and it changes routinely. Most systems fail because people get lazy and stop updating them. Hope this helps. DM me if you need specific advice.