Where can I learn how to manage a custom wordpress site?

Checkout is a high-quality WordPress theme by Array built for marketplaces of any kind. https://arraythemes.com/themes/checkout-wordpress-theme/ Let me know if you have any questions about implementation, and I'd be happy to setup a call.

Well first off I wouldn't recommend trying to power your client's database with the standard WordPress database instance. They can use WordPress for the website all they like. But with millions of records like that, you really don't want to move those data in with WordPress. For that matter, you're probably gonna want a dedicated database server to answer those questions, if the client doesn't already have one. You might also need to investigate something like Elastic Search, depending on what your search requirements are and what sort of database they are using (pay close attention to whether or not it provides full text search ability). Beyond that, and somewhat obviously, you're also gonna want SSL on the website and to make sure you're up to date on federal and state privacy requirements for data like this, as well as best practices for securely transferring data over the web (port configuration, ssh keys, etc). The privacy concerns would raise a flag for me. If those data have anything personally identifiable in them - which they very probably do - then you're really gonna want to make sure you do your research on security of personally identifiable information. Government specific privacy /security /auditability requirements may also apply, so double check that too.

Yes. https://docs.woothemes.com/document/configuring-woocommerce-settings/

Be careful when simply wrapping things into a mobile app. This can backfire on you and you could be presented with some very harsh edge cases. PHP isn't going to be encapsulated so much as the HTML/CSS/JavaScript. So keep in mind your mobile application could likely be in a position of requiring an online connection to work (because it must interface with your existing hosted WordPress site). This could also mean your hosting solution needs to be evaluated to ensure you can handle any increased traffic (and those traffic patterns could be different when coming from a mobile app that perhaps loads things the user doesn't see right away, accesses content that may not be cached, etc.). You want to ensure your server doesn't go down because then your mobile app would be "down" as well. That said... Things like Phonegap (web views) are a wonderful idea for utility apps because the performance is good enough for those (and hey even some games) and they end up being easily ported across many mobile operating systems. I would look into Phonegap, Appgyver (a new and totally awesome one because parts of it will utilize the native OS and your app will feel more responsive), Appcelerator Titanium, and perhaps even the new Famo.us one. Also take a look at the Ionic Framework for some further ideas about mobile UI and what you can do with these web view style apps.

It sounds like you have plenty of skills to get started now. There's no need to keep re-training in different areas when you have experience to get started today. My suggestion would be to pick a niche and try and become the go-to guy in that particular niche. Let's say, for example, you are interested in men's fashion. You have experience in creating Wordpress ecommerce sites. You could call up maybe 10-15 of the local businesses in that niche in your local city/state and offer to make their website and get them in on a set-up fee and then a monthly maintenance retainer. This approach would be lower stress (because it's something you're interested in) and also because you could create a methodical framework that you could apply to other businesses in that niche. That's just one idea. Second idea - create a course on WooCommerce development and put it on Udemy (or Coursera etc). Note down 10 of the biggest obstacles you've had to overcome when building sites for friends and family and then note down 10 of the most important considerations people should consider before people get started. Now you've got 20 video lessons for your course. Charge for the course on Udemy or use it as a marketing tool to get more b2b development work. Idea 3: Go make money on freelancer.com, peopleperhour etc. Perhaps you've tried this already? Skills like yours are in demand on those platforms. Idea 4: Take the things I noted in the second idea above, and turn it into a handbook. Sell that book via Amazon. Idea 5: Go on Tweetdeck. Create a column that searches for people who are using keywords like "Wordpress woocommerce issue" "Wordpress woocommerce help" "WordPress woocommerce problem". Give them your clairty.fm link and tell them you'd be happy to have 5 minute discussion to see if you could help them resolve their problem. Idea 6: Find 10 major theme development companies. Sign up to their help or support forums. Do a similar thing to what's noted above on Twitter and offer to have a quick call via clarity.fm to see if you could help. Idea 7: Go down the route of finding existing Wordpress/Woocommerce blogs. Write posts for them about specific WooCommerce issues, problem solving or project management tips. Do this with the aim of improving your inbound consulting gigs. Idea 8: Do the exact opposite of whatever those friends are telling you. Idea 9: With your skills you could easily start a dropshipping company. I won't go into all the details here but just start looking at sites like Clickbank or Product Hunt to get a feel for something you're interested in. Build your site and start dropshipping products. https://www.woothemes.com/2015/06/dropshipping-beginners-guide/ Wordpress consulting alone, yeah it's probably quite competitive, but that doesn't mean there aren't plenty of opportunities for revenue. I think you will be even more motivated, successful and less stressed if you pick a niche industry, product or service to focus on. Enjoy it!