Firstly, look at the job boards, JobServe, monster, JobStreetAsia, as appropriate and see how many jobs there are that match your skillset, just as if you were actually looking for a job. The look at the ration of permanent to contract. Of course, the number is important. If you are in a truly niche area, then it might be safer to stick with permanent employment. You could set up a consultancy firm, but that might take a very long time to get off the ground, so I would say that you should stick with perm//contract for now.
Next, are there any contractors where you work? I have had quite a few perm staff approach me quietly over the decades, asking about contracting.
I give advice, leave them to decide, and my lips are sealed. The best way to learn is from the horse’s mouth.
You could also try talking to a recruitment agency. Be aware that they are trying to sell you something (their services), and might even give advice based on what they currently have on the books, rather than the true picture. Perhaps if you say you ar considering a move in 6 months an asking for general advice as to perm//contract. Despite the possible disadvantages, no one is better situated to know the job market thanan experienced recruiter.
You are, rightly, concerned about your mortgage – does your partner work? If not, do you have savings? If not, can you save enough during an initial contract to tide you over rough times? After a few years, this should be no problem.
Btw, in my experience, contracts are always for six months, but I always ask how long the project is supposed to last . In my line, that’s generally 18 to 24 months.
Will you make more money? Certainly at the top line; you should at least double your salary. As has been discussed here very often, so I won’t go in to details. You will earn a lot more, but pay have no paid holiday or sickness (family holidays will effectively cost you double); you will also have to pay employer’s social security, in addition to employee’s. I advise a Ltd company. You will pay for an accountant, who ought to save you ten times what you pay for him. At the end of the day, I can say each year the equivalent of my permanent colleagues’ annual salary before tax & dedications. Of course, YMMV. And I am in an area where I have little trouble finding contracts; perhaps a total of 6 months over *cough* decades.
This is just a mechanism. Your question is, at the end of the day, subjective, and only you can answer it.