Definitely more of an art than science. Having been thru one growth investing cycle and preparing for another, I can't point to any advanced metrics or quantitative frameworks I used to make the decision. Just simple ROI math and also have a strong view on your runrate EBITDA margin (i.e. industry averages and adjust for your biz's idiosyncrasies). Also, trust the process -- riding the j-curve down is unsettling but necessary in order to unlock higher levels of earnings in the future. This is why there are so many businesses with ~$500k of earnings in this space -- many founders / sellers are unwilling or unable to take this leap.

You should focus on being positive cash flow. You can cut expenses and grow at the same time. What to cut and invest in depends on how much actual operational leverage you get in the business and the mix of fixed and marginal costs for your services. I would suggest to do a hybrid role to someone that is a high potential before adding any additional SG&A.