As with everything to do with marketing, the answer is 'it depends'. Not much point in putting ads for an enterprise ERP system on FB.

Google Ads is complicated. Avoid it unless you are prepared to put in serious time learning how it works.

I tried Twitter ads a few years ago. Their demographics (knowledge of their customers are) was total garbage. Waste of money.

In general, do lots of small experiments and measure the results (clickthroughs, time on page, sign ups etc). If you can't measure anything meaningful, think carefully before handing over any cash.

I am not an expert, but any advertising >>> no advertising. I would go with your first instincts, slap some cash down in a few different places, and start refining later.

he's not talking about ADVERTISING. You need to reach out to people yourself, cold email, tell them you have a product that does XYZ and think it might solve their problems.

I would say, don't spend a dime before you read at least one marketing book. I'm now reading "Marketing Made Simple" and so far it's very insightful.