at a (fairly modest) 3.3 its like 1100/month.

I don't know what kind of rates are available to non-profits, but with 400k in hand you can find nicer rates than 3.3 (as of today, at least).

that covers quite a few colo possibilities.

USD money market funds from Vanguard pay about 3.7% now. Personally, I would recommend a 50/50 split between a Bloomberg Agg bond ETF and a high-yield bond ETF. You can easily boost that yield by 100bps with a modest increase in risk.

Another thing overlooked in this debate: Data center costs normally increase at the rate of inflation. This is not included in most estimates. That said, I still agree with the broad sentiment here: 400K USD is plenty of money to run a colo server for 10+ years from the risk-free interest rate.