Blogging and knowledge sharing, plus a little bit of marketing of posts will get you noticed. If possible, also give talks at Meetups or conferences.

Articles that require arcane knowledge, but shows demonstrable value (saves customers time, money, improves /dev experience) will set you up as the authority. You can also do tutorials, walkthroughs comparison posts (x vs y).

Next step is to contact company X and share your posts, and see if they need professional service help. If the product is early stage, they can probably handle the work in-house. If the product is medium mature and has traction with enterprise, there's a good chance they can't handle all the customer demand and will send them your way just to keep the customer happy. If you get to that stage, may want to keep an eye out for other devs you can bring in on larger projects.

Find a conference that specializes in your tech. Network like crazy, hand out cards, and give a talk if you can. You want to build a personal brand with your most interested potential customers. Be prepared to state how much you would charge and to do some back of envelope scope assessments. Follow up with the people you met after the conference is over. Organize a tech meetup in your town too.

Do I just need to start cold calling companies that based on their name “might” have this software as well, and explain my case?

So, as someone who works for an MSP that specializes in a certain vendors products, it's maybe worth checking out the websites of who may be your competitors. They love to put their clients names on their websites. This should help you at least start building a list of companies to target (LinkedIn is heavily used by sales peoples these days).

I'm not a big important decision maker but if I always came across your website when searching for answers about software X and your GitHub when looking for code for software X I would go to my manager and say "why don't we just pay this person to sort it out, they seem to know their stuff" and there's a fair chance we'd do it.

Do I make a website pretending I’ve been doing this for years, then send out an email being like “Y consulting is now accepting new clients who use software X, check out some of our most popular custom reports, then fill out this form to get a quote for one of your own!” to just every person in the industry and hope someone actually reads the email?

As a site manager for a smallish international, pleas don't email me with "I am X, and can help you with Y". You will be thoroughly ignored.

I good website or blog with good info and a services available would be of more value to build a reputation, as @fubarx@lemmy.ml mentioned

You could contact a larger contractor. Going as a subcontractor to a larger contractor means that you get access to their sales department and you can utilize them to build a name for yourself. The downside is, of course, that they take some of the earnings.

As others have mentioned - I would second. A good website. Let them come to you. Give your solutions to common problems. Create a github. Provide repeatable examples on your GitHub and encourage contact for custom solutions.

This won’t be a multi million dollar business. At best you’ll give yourself some work to get your name out. Companies don’t talk to each other - but maybe your niche is different. This is really the only path I can see without attaching yourself to a larger entity.

Find a conference that specializes in your tech. Network like crazy, hand out cards, and give a talk if you can. You want to build a personal brand with your most interested potential customers. Be prepared to state how much you would charge and to do some back of envelope scope assessments. Follow up with the people you met after the conference is over. Organize a tech meetup in your town too.