I have this exact issue going on now as well.
We moved double six digits out of out a single tech stock that accumulated due to RSUs, the idea was to diversify and take advantage of a price bump that happened last week. Now the funds have settled and I need to reinvest them but the feeling that we're in a bubble has me hesitating to buy back in. I know intellectually that the market is at an all-time high most of the time, I know that its best to keep them invested but the temptation to just go ahead and pay off the mortgage for the perceived security is real (I've been a member of the don't-payoff-your-mortgage-club for a while).
These funds are about 18% of our LNW and 13% of TNW, we're about 5-7 years out from FIRE so probably enough time to recover from a COVID bubble if/when it happens. Maybe I'll just DCA it back in over the next month or two, somehow the decisions are more difficult with more zeros involved.
As an aside, did you know that Schwab and Fidelity have a max $100k daily transfer limit? It might be an online limitation, I didn't call in, but online that's the max in a 24 hour window.