"3 months to build authority" - where exactly are we getting that timeline? A case study from someone with a pre-existing audience? If you're starting from zero, three months is barely enough to figure out which channels even give you positive ROAS.
i'd push back on "build an audience first" without defining the terms: an audience of 10k bots who never convert is worse than 500 real decision-makers. Test multiple niches from day one, but track attribution properly. Set up UTM parameters, use a proper CRM, and cap your ad spend per niche at £200-£500 until one shows a CPA that actually pencils out. Anything else is just paying Meta for vanity metrics.
that said, if you're not willing to analyse the first six months of data honestly, pick one niche and pray. Most people burn budget on "authority" that doesn't move a single pipeline metric.