I think it is a bit of a stretch asking for a 5 year multiple entry visa for a single annual medical appointment but give it a go and let us know how it works out :-)
Currently you would be liable only for assessable income brought into Thailand. There are also 63 Double Taxation Agreements in place. You need to look at the agreements that concern you.
In Pattaya Tik Tok Services can help you open a bank account on any visa ie even Visa Exempt. cost is about 4 or 5k THB - forgot which these days. There are other agents available but most use the same bank.
All major IT projects in the UK seem to have major issues too. Bent Flyvbjerg at Oxford Uni has a megaprojects database which covers all of these project failures. The record of successful project delivery is abysmal. I was a Global Portfolio Director at a global company. Seen it and been part of it :-)