This supposed overhead is not worth >300 thousand dollars.
If I saw a founding member trying to divest himself at a 20% discount in a funding round for petty cash, I probably would find something else to invest in.
Not to you, but you don't matter. The investors and the other shareholders matter.
If the company is doing well in the eyes of the CEO, the remaining shareholders and the investors this is their chance to get a larger slice of the pie at a discount.
Founders are not by law or the shareholder agreement required to stay with the company across its lifecycle, and founders being bought out during funding rounds is common and does not immediately lead to investors bolting from the deal when presented with a good enough story behind it (such as someone wanting to move on, or being tired). It's actually quite rare to see a 5 year old company where all co-founders are still active and in their original roles. Nothing worse for a company than to have a bunch of ex-founders who no longer produce anything and that are holding on to sizable blocks of shares and that do not wish to sell. In investors parlance that's called 'dead wood' and the more you have of that the harder it will be to raise money.
Now if all of the founders want to take money of the table that is known as a buy-out and tends to be looked at differently, but even those deals are made, usually at some kind of discount. The version where other employees of the company elevate themselves to the C level is called an MBO and is also quite common.
Optionally there will a bank involved to fund part of the operating capital of the company depending on the amount and liquidity of the companies' assets.
Source: veteran of a bunch of deals involving my own company and being closely involved in another 50 M&A or investment deals or so, usually on the investors / buyers side.
If I saw a founding member trying to divest himself at a 20% discount in a funding round for petty cash, I probably would find something else to invest in.