Here’s my take on your questions:
“Anyways, my question is whether I should I invest all the money I'm socking away right now, or keep a big chunk in cash to invest slowly over time post-FIRE.”
“Should I be holding back a chunk of my pre-FIRE savings in cash so that I can gradually put it into my AA over the course of my post-FIRE years? If so, how much?
How are you thinking about this? What does value-averaging look like post-FIRE when you're not adding to the kitty anymore?”
So, for me, I have taken the approach of targeting a specific amount ($1M) in our AA using Value Averaging strategy to get to our early retirement transition. I have no intention of continuing any value path contributions post-FIRE. It’s about survivability of our nest egg from that point onwards, as we plan to scale back to part-time work and supplement our income with modest withdrawals from our AA to support our lifestyle. Indeed, the $1M target for us as an early retirement goal number is based on our lifestyle needs and projected part-time income (using FIREcalc model, etc.) after transitioning to early-retirement.
The answer to post-FIRE value path contributions is really a question of risk tolerance in my view. Bernstein covers this topic pretty well on pages 154-157 in “The Intelligent Asset Allocator” under the ‘Executing the Plan’ section of Chapter 8 when he covers value-averaging.
If you have the cash to plow into your AA, my view is to align your value path plan for getting the cash invested as rapidly as possible to achieve your FIRE goal. If you have a high tolerance for risk, and don’t mind the volatility, no need to hold back any cash from the value-averaging plan. Get your $100k in immediately and plan out quarterly or monthly or bi-weekly or ?? contributions for your ~$200k per year until you hit your target of $2M. For me, I only keep a cash safety cushion of $20k, the rest goes into aggressive value-path plan.
As Pb4uski suggests, you can take whatever cash you have on hand and “new money to bridge the gap” you have to add to your AA now to target a 60/40 split by the time you hit your $2M target. With this strategy you have an AA that is probably better for post-FIRE stage in life. When you hit value path target is $2M by end of 2018, you can shift to a survivability model (using FIREcalc model, etc.) to enjoy the lifestyle you want to have post-FIRE. With the right amount in your AA nest egg and the right risk profile, you should be comfortable with no significant additional contributions to your AA or value-averaging to speak of.