You have valid reasons to start early. However, redo your math calculations. The biggest unknown is how long you will live and your quality of life. That's part of the reason why there is no right or wrong answer. My goal is to have enough money in my 401K so I can live comfortably in retirement with only my own 401K. If the pension checks do come to me so much the better. If SS checks arrive monthly, so much the better. However, my faith in wall street protecting the pension system and congress protecting social security is not too high.
As to your estimated investment of 400K after 8 years, I'm showing closer to 336K after 8 years not 400K. Also, that is assuming that 100% of the SS check you said you would get goes to investments and nothing being taken out. On the other hand, I also assumed no increase in the SS check also over those years. My method was to do 96 total calculations showing what the value would be of the first monthly payment showing 8% annualized returns compounding monthly. I repeated 95 more times, with 1/12 less of a year of time of interest accumulated. Then summarized all those 96 rows (months) to get to the final. There are other calculators out there that will give you similar numbers. Also, in a "short" amount of time of 8 years in the stock market, it could be very good (much greater than 8%, or very bad with negative returns). I don't want you to go and planning on 400K and finding out only 336K was in there with the market showing the 8% gains you were anticipating.