I did exactly what you are proposing but my circumstances were based on redundancy and being fed up with what I was doing.
I was a power station mechanical engineer (MSc and MIET) and project manager for over 30 years and I found getting the Oftec certification very straightforward (=easy).
I would recommend going on a couple of manufacturer's 'free' courses as they give you all the fault finding 'knowledge' for their kit and a little experience in the training centre. The rest of it will come on the road and if you're anything like me then your problem solving skills will already be well tuned!
Some manufacturers combine their own courses with the Oftec course/assessments and these are particularly good value both from a learning perspective and value for money.
What I did find when I started was that most people lie about when their boiler was last serviced...so assume never..... and they don't tell you that they have just run out of oil and dragged all the muck at the bottom of the tank into the burner pump and nozzle! (Or indeed that the local lads pi*s in the tank after a good night out!) I even had one customer exclaim that the last 'engineer' had never taken the burner apart to clean it and had never changed the nozzle in 20 years! I could tell , it was stuffed full of cat fur!
You will very quickly realise that a small range of essential spares 'on your van' is worthwhile as most faults are caused by the same, but different makers, parts. You also need to find your local parts supplier preferably very close by. I found Shoreheat very good but I was near their main warehouse! You will need a good quality combustion analyser that will need annual recalibration (best in the summer when it's quiet).
You will come across all sorts of manky old obsolete boilers which you would rather not touch but do so because it's good for business.
You also need Liabilty insurance just in case you wreck someone's house!
The richest customers never pay at the end of the job usually because... 'my husband has the cheque book today'.... oh yeh who are they kidding!
I'm not sure what value you will get out of the plumbing course unless you know nothing about domestic heating as you will find it very basic after your current work. Although basic heating control system knowledge is essential but you can get that from a book, the Oftec books are very good. I hated the plumbing bits and tried to avoid them and just concentrated on the boiler servicing and faultfinding which I really enjoyed. I progressed onto oil fired cookers but some of them are an abomination and very difficuklt to work on.
I ran my business for 5 years until ill health stopped me in my tracks but I enjoyed doing it from the perspective of learning a new skill, getting qualified and starting my own business. I would still be doing it now part-time if it wasn't for the cost of the combustion meter recalibration and insurance, accreditation, etc.
Basically what I'm saying is because you are a trained engineer you will very quickly learn the ropes and be successful. After all you are probably more capable and better qualified than many 'engineers' doing similar work. (Stands back and waits to be shot down in flames!
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Go for it and good luck!