Hi
First of all - good luck.
Second, the TAM is less important, the SOM (relevant Share of Market) is the important piece of the TAM, and this is much smaller. Either way, not everything that shines on paper shines in real life, or as mike tyson said: "everyone has a plan until they get punched in the punch" - in this case, business plans often don't survive reality.
1. My advice: unless your service is something that already exists in the market and has been proven to work, I would wait with the business plan (BP) and instead create a Business Model Canvas. If you want to write a BP nonetheless, I would be happy to review it for you (I even teach a course on it).
2. Stage 2: create an MVP / validate your idea. I am happy to explain to you exactly how to do this. Doing this properly, will save you a lot of time and money down the line. [see more info on validation in one of my previous answers: https://clarity.fm/questions/6423/how-do-you-do-market-research]
3. Improve MVP + create deck + write BP (only if you need investors which I assume you do because you mentioned the deck). I can happily give you comments on your deck, refer you to someone who creates beautiful decks at reasonable prices, and help you prepare your pitch via online video chats.
4. Create (almost) final version of product.
The MVP is the most important stage. It doesn't even have to be a working product to first check the demand in the market.
I've successfully helped over 300 entrepreneurs and would be happy to help you. After scheduling a call, be sure to please send me some background information so that I can prepare before - thus giving you maximum value for your money during the call itself.
Good luck