This is another example of what Richard Thaler calls "mental accounting." (
https://en.wikipedia.org/wiki/Mental_accounting) It is a useful tool IMO but it is easy to overdo it.
Money is fungible. All of your money is your (sole) portfolio. Planning the liquidity of your portfolio to suit your expected spending that is really all that is needed. Money that is not needed for 5 or more years is probably best invested in equities. More near-in needs, say 3-5 years might be covered by very conservative equities or by fixed-income investments of some kind. Very near-in needs militate towards t-bills, CDs, and maybe a little bit in fully liquid money-markets. As an expected need moves nearer and nearer on the time horizon, its most suitable investment vehicle changes.
I use this approach in general (very similar to OldShooter's comments), to balance my needs for flexibility, liquidity, risk:
If possibly/likely needed in <5 yrs - MMA (I calculate the amount based on known/expected needs. It's currently ~4% of the portfolio).
I keep another pot for expenses the might be needed in 5-10 yrs in a ~50% equities portfolio (This is an arbitrary amount. I don't really have any 5-10 year needs identified at present. ~25% of my portfolio is currently in this easy to access, relatively stable bucket).
All the rest is considered for "growth" and is (for 10+ yrs needs) in a closer to ~65%+ equities portfolio. (This is currently ~70% of my portfolio).
Overall, my portfolio is ~60% equities, but I split it up in the hopes that I will have less volatility in those segments I might need to withdraw from in the nearer term.
As for replenishing/more directly to OP's original question, I budget an annual amount to add to the MMA, to account for changing needs and accumulate enough for any large, unexpected HC OOP expenses. If the MMA balance is more than needed, I transfer the excess into one of the other investment accounts so it's put to work.
Obviously, if I just kept it all together, and did the mental accounting thing, I could sell/rebalance when funds are needed and it would work out fine; I just like the "sleep at night" feeling the liquidity profile provides.
YMMV; both ways can work equally well.