I'm keen to see what responses you get, and where you get to with this plan as my own tentative post FIRE plan is along similar lines, ie to qualify as CPA (currently living in UK and hold an equivalent local qualification, eligible for green card via US wife) and work seasonally doing taxes to top up our income.
I see the same issue as you, namely that the financial advice industry does not seem to be well aligned with the interests of investors. Of the services that I've seen they seem to range from the "charge 1.5-2% of assets and invest your money in underperforming high cost active funds" at the worst, to "explain why most people are better off with cheap trackers and invest your funds in cheap trackers, but then charge you 1% of assets to hold your hand" at best. In spite of all this most regular people I speak to, ie not on this board or bogleheads, really do seem to feel the need to have someone to hold their hand.
My thinking with this is, as ScooterKC says the actual input they need in a given year is likely pretty low and limited to initial setup and preventing them from doing anything stupid. Your challenge is that in a charge for time model your fees are very visible where as costs taken front their portfolio are out of sight, out of mind. The idea of combining this with a tax practice seems like it would be beneficial as you have already established a relationship of trust, and already established the mental concept of them paying you for your time.
If you can target high net worth clients and charge them a fee which is way below the percentage they are currently paying a traditional advisor you may be able to get a decent income, but fundamentally the problem seems to me that the conventional industry overcharges for a service that, if you charged more fairly probably wouldn't actually be all that lucrative.
The answer to me seems to be to get to a fire position where you are no longer reliant on taking money from your clients to pay your bills, thereby removing the conflict of interest which is otherwise inherent in the industry. You could focus on less profitable but more satisfying cases, like getting to new graduates early on and helping them into the path to FIRE, and view it as satisfying work that makes the world a better place, while the income you do generate is a nice addition rather than a key driver.
Interested to hear any responses or rebuttals to this idea