> Do you think there's enough demand out there to have a company that does just technical (e.g. software quality, security) due diligence?

My company (used to, we've grown to do other things) only do Tech DD and we've done really well, so the answer is yes. According to Pitchbook there are about ~3,000 software company acquisitions every year - so that might tell you how much potential business there is.

There are consultancies who provide these DD services certainly as a line of business, if not their entire business. Like any other professional services enterprise, a lot of it depends on your network and ability to generate new business, especially in the early days. If you're not in the position to do it yourself, perhaps there are opportunities to get involved on a contractor basis with firms that do.