I did max out my 401K contributions in order to get to $24K before retiring at the end of last July. I wanted to go earlier, so actually set my contrib to the max my company would allow, but then got talked into staying. Given the behavior of the market during the latter part of the year, this turned out to be a good thing and I'm glad I did it.
I can't answer your question about combining the 401K and Simple IRA in the same year, but it seems unlikely that the combined limit would be higher than $24K. I'd guess that you're better off putting the max into the 401K and then rolling it over after you leave.
My income level has been such that the 401K has been the only tax-deferred option available to me for many years. Even once I FIRE I will still be executing remaining stock options for several years and the income level from that will still put me over the regular or ROTH IRA limits.
Unless you or your DW are going to have compensation from working (not sure if your stock options or the business you own will provide that), you won't be able to contribute to a regular or Roth IRA in the future at all. You can contribute $6500 to a tIRA this year, but it won't be deductible.