The first bit of advice I would offer is: don't chase returns. Chances are good that by the time you hear about a no-lose opportunity, thousands of people have passed on it. You have approximately 11 years before reaching your full retirement age. I would divide my investable money up, putting between 70% & 80% in a fund that invests predominantly in strong stocks with good growth potential and steady earnings. I would take the balance and consider investing in some more risky investments, making sure you are still somewhat diversified. At those levels, even if your risky investments tank, your traditional portfolio should grow somewhere in the 5% to 7% range annually, which would come close to doubling your initial investment.
Chances are good you will live longer in retirement than your remaining time as a full-time appraiser. I would try to control your expenses (within reason) and invest as much as possible as soon as possible to capture as much compounding as possible. Depending on health and family history, hold off on filing for Social Security as long as you can, to both build up your earnings and to take advantage of the lifetime premium granted for waiting. In your case, holding off until your 70 would increase your payout by at least 24%. If health and circumstances allow, I would continue to work part-time so that you can continue to invest some and not needing to draw down on your investments until you decide to fully retire.
With the various changes coming to the appraisal industry, I would continue to expand your non-AMC work and strongly consider obtaining your Certified General. I know obtaining the required hours can be difficult, but I have to believe you have already completed some simpler non-residential appraisals, and over time you should be able to pick up additional qualifying assignments. Even if you receive a commercial assignment you are not qualified for, chances are good you either have or are capable of developing a relationship with a CG, that will work with you when you bring in our own assignments.
Over the years, my retired partner and I developed a relationship with a CR who is located about 70 miles to the north. The relationship began 30+ years ago during a CE class break. We concentrated primarily on commercial and high-end/odd ball residential (none AMC), and he did mostly high-end residential. Our areas of concentration have changed little over time. Our general service areas are served by different MLS systems, but occasionally we will each receive assignments near or in the other one's service area, so we will reach out to each other regarding market and comp information. If he receives a smaller commercial assignment, we will work together, with him doing most of the inspections and report writing, and I concentrate on building costs if the Cost Approach is used, reviewing the Sales Comparison Approach, and then preparing the majority of the Income Approach. I also review and sign off on the finished report. He then delivers the report to the client and sends me a check based on my time and contribution.