I've been working on an open source search engine for a few years (https://github.com/typesense/typesense) that's far more easier than Elasticsearch for a number of use cases and is a great alternative to Algolia.
I was faced with this same dilemma when I started out: how do I protect future commercial interests without stifling the spirit of open source? Eventually I just chose good old GPL and decided against BSL-like licenses for a number of reasons.
GPL is a fine balance. When you are small, your greatest risk is not an existing or future competitor: it is death by obscurity. To that end, you don't want to slow down adoption by choosing a license that screams "$$$" or makes somebody's legal team uncomfortable. In any case, if a big company wants to copy you, they will and can with the resources they can throw at it.