At WP Engine, everything in marketing and sales is included in CAC. Salaries, commissions, coupons, direct advert spend (which you're saying you don't yet have), fees, travel and other costs associated with conferences, etc..

My advice is to err on the side of putting too much in CAC, because that helps you honestly understand the costs. Ignoring some costs just because they don't scale with company size or marginal new customers doesn't make sense to me, it simply means that certain components of your CAC you expect to get more efficient over time. Indeed, they had better! So measure it, instead of ignoring it.

You also might find that some of those direct-spend channels are not as inefficient as they seem compared to things like SEO efforts. Or the reverse! All good things to explore of course.

I'll also note that at $19/mo in the crowded space of CMS offerings you will find that very few channels will be efficient compared to the revenue you're generating. It sounds like you know that, and are dealing with it with "scalable" efforts like content marketing, however again you should be ruthless in understanding how those costs are really translating into orders and whether that's a financially sensible total strategy.