Most requests I get for marketing help start with which channel to add. That is almost never the binding constraint. The constraint is usually that nobody can say what the current spend produced, so every decision after that is a guess wearing a budget.
Fix measurement before you increase spend
If you cannot trace an enquiry back to the campaign that created it, adding budget increases the size of the number you cannot explain. Before anything else, get conversion tracking configured properly in GA4, make sure form submissions and calls both register as conversions, and tag your campaigns consistently so the source survives the journey.
This is unglamorous and it takes a day or two. It also routinely reveals that one channel everybody assumed was working is producing nothing, which pays for the exercise immediately.
The funnel leaks before the channel matters
A page that takes eight seconds to load on mobile data will waste any traffic you buy. So will a contact form with eleven fields, a phone number that is not tappable, and a WhatsApp link nobody put on the site despite it being how most local customers prefer to make contact.
I would rather fix those four things than add a channel, and the arithmetic supports it. Doubling the conversion rate on existing traffic costs less than doubling the traffic, and the improvement applies to every channel at once.
What each channel is actually good for
Paid search captures demand that already exists. Somebody is looking for what you sell today. It is the fastest route to measurable return and the easiest to overspend on, because a broad match keyword will happily consume a budget answering questions nobody asked you.
Social advertising creates demand rather than capturing it, which means longer payback and a heavier dependence on the creative. Judge it on cost per qualified enquiry over a quarter, not on engagement in a week.
Search optimisation compounds. It produces very little in month one and can carry a business by month twelve, which makes it a poor fit for anyone who needs revenue this quarter and an excellent one for anyone planning past it.
Content earns attention and then has to convert it. A useful article that never mentions what you do and never offers a next step is a public service, not marketing.
The numbers that decide the budget
Cost per qualified enquiry, not cost per click. Enquiry to customer conversion rate, which usually says more about sales follow-up than about marketing. Average value of a customer, without which return on ad spend cannot be calculated. Time from first contact to sale, because it tells you how long to wait before judging a channel.
Four numbers. Most businesses I work with have none of them at hand when we start, and the act of collecting them changes where the money goes more than any channel recommendation does.
One quarter at a time
Pick the two channels closest to how your customers already find suppliers. Run them properly for a full quarter with tracking in place. Cut the weaker one, move its budget to the stronger one, and add a third only when the first two are producing predictably.
It is slower than a campaign launch and considerably cheaper than a year of spend nobody can account for.