What is you safe withdrawal rate, current asset allocation, and age? It sounds like your planned final allocation is 80/20. Is that right? Do you have any fixed income now or coming in the next 10 or even 20 years. Also do you have a mortgage? My advice will change depending on your answers.
Historically, paying off the mortgage is your best hedge against sequence of return risk. If you have one and it has a crazy low interest rate I can see why you might be interested in alternative ways to hedge against sequence risk.
True market timing is gambling as some have pointed out but that is not what you are doing. Hedging against sequence risk is smart when market valuations are quite high at retirement, especially if your initial SWR is >3%.
I am personally not as confident as you that another 20-30 % drop is unlikely. Economic fundamentals seem strong today, but equity valuations are still historically inflated (Shiller PE of 27.5). A recession could easily arrive in the next couple years and take markets down quite a bit further.
I personally like the model in the below link for an active rising equity glide path from a 60/40 to a 100 percent stock allocation if you are looking at a 50+ year retirement without substantial other sources of income.
https://www.earlyretirementnow.com/2017/09/13/the-ultimate-guide-to-safe-withdrawal-rates-part-19-equity-glidepaths/But managing the glide path seems like a pain and it sounds like you may never be comfortable with 100 percent stocks. If you want to stick with something more like 80/20, consider lump sum investing to 80 percent stocks, keeping 3 years of cash, and putting the rest in bonds. Then spend the cash buffer only if your investments are down more than 20 percent and dont replenish it.
https://www.earlyretirementnow.com/2018/05/23/the-ultimate-guide-to-safe-withdrawal-rates-part-25-more-flexibility-myths/