Indeed having money available is great.
My largest bill however is my mortgage, ive got approaching 6 figures in my overpayment reserve so can ask for that to be offset against needing to pay anything monthly. In theory for about 10 years

Most now dont, but many older mortgages allowed for overpayments to be drawn back down.
Money overpaid is not necessarily "lost", you need to understand these terms as well when you take out a mortgage. Many don't do anything but look at the headline.
You need to look at investments for more than 12 months, thats very high (10% it includes COVID bounceback period).
IMO balance is the best thing, you should have immediately availble funds (circa 3 months). longer term funds, plus for me paying off the mortgage and lowering the monthly outgoings.
And if its in regards retirement planning then you should be paying more into pensions than self investing (assuming your not capped), its far far more effective.