Put it in a Vanguard money market fund for safekeeping now and then transfer it into either their 500 index fund or their total stock market portfolio fund - 8% per month over the next 12 months. Reinvest the dividends and ignore it until/unless you really need it. If you mess with it you realize the capital gains which - while still better than paying regular rates - you don’t want to do if you can avoid it since unrealized capital gains sort of wind up compounding tax free until they are realized.
It’s not as good as a 401k but if you’ve exhausted your other more Tax efficient options it’s reasonably good and you can pull the dividend income out (which you will be taxed on continuously to cover the taxes generated if you need to.