Training to become a gas engineer in my own time?

[Oil&gasman]

Hi everyone I currently work in oil and gas industry working in North Sea and further afield operating on high pressure oil and gas plants. This line of work means I have on average 6+ months off a year, im thinking of training to become a domestic gas engineer in my time off to earn a bit extra if I want too and also diversify my income if I become Ltd company (IR35) as I often get seconded to projects but as things stand if I do any extra trips offshore I’m looking at a week minimum away from the family which I would like alternative options.

I’ve seen lots of stuff online about courses to do which allows you to become a gas engineer in X amount of time etc but I don’t want to do anything that’s not required or pointless can anyone assist with letting me know which courses I need to complete and what to avoid? I’ve got 2 heating engineers who agreed to let me work for them for free to build up a portfolio