Hi Ray,
My background originally was in physics and real time interactive simulation (videogames) so when I first started getting into SEO years ago, I was also neck deep in trying to offload a fast fourier transform onto the GPU to try and speed up my water simulation. In other words, I tended to look at the technical side and picked up that stuff relatively quickly. I wouldn't say I'm an expert at it, but I know my way around html and CSS, PHP, MySql, even a bit of python (that I've since mostly forgotten).
So, here's the other side of the fence: was that investment of my time worth it?
Your most valuable offering if you're going to be a local SEO consultant, is your roadmap, and your ability to diagnose and troubleshoot. There are a lot of tasks that need to be done (troubleshooting GMB profiles, backlink building, citation cleanup, review building, onpage optimization, keyword tracking and other ways of measuring progress, potentially relationship building with industry editors and other backlink opportunity gatekeepers, etc) and html/css is a pretty small piece of that puzzle. Even if you learn the basics, you're going to encounter clients using Wix, clients using Joomla, clients using some horrendous mess they cobbled together themselves, etc. Knowing just enough to think you can do it yourself is a good way to waste your time, and sell them a subpar job that's going to embarrass you if they ever ask someone really in the know for an opinion on the quality of your work. if you're a kick ass local SEO but you did some shoddy coding for them, it's going to make them doubt your work as a whole if they hear that from someone they trust. It's not just about design after all, good code is elegant, easy to update, etc. For modern responsive sites, that means getting up to speed with some of the frameworks out there to help, like bootstrap, sass, compass, and so on. Getting on your A game is going to be a real learning curve, and you're going to need a mentor to get your work not just functional, but clean. Coding conventions are a mess on the internet compared to what you see in a 'real' dev studio... lot of self taught people doing 'what works'. Don't be that guy.
If I could go back and skip all that stuff and focus elsewhere, and found a good outside resource to fill in the technical areas for me, I would have saved myself a lot of time. Trying to be a jack of all trades is, in my opinion, a good way to limit your income, or at least slow down your growth. On the plus side, if you did put in the time, you'll at least be in a better position to assess someone else's skill when you are ready to expand.
All that said, I do think it's worth it to know the basics, and some of that can at least superficially look pretty technical. Google Analytics referral tracking for example can be thrown off if you have an http site, and if you don't know a little bit about how that information is passed in the first place, it might seem like an unintuitive random fact. The actual technical stuff you should spend time learning though probably isn't html/css.
tl;dr - unless you actually like being a jack of all trades and want to add web design onto your list as a service (after hundreds of hours of learning curve that you shouldn't do on a client's dime + the effort of finding a mentor to look over your work and help you get up to speed) you should stick to your core value you're bringing to the table: SEO. It's perfectly fine to tell a client with an unacceptable website that they'll need to get it fixed, but that that's not something you do. You should have a referral network built out though, so you can point clients towards a good resource.
