MW. My use of recent was a relative term. In the case of swimming pools you are, of course, quite right that all sorts of problems might appear at a later date. However the quality of work and the attitude of the builder should be a good guide to his worth. Having said that nothing in life is certain and that is particulalry true of the Building Industry anywhere. Recent work is necessary, because his tradesman may have changed since earlier work and so might his professionalism. The whole subject needs a book really and it is quite difficult to get the right approach defined in a short post.
It is also true that there are good tradesman in BG, but it requires a lot of effort to find them. Then, as you do, strict supervision is more or less a must. All very difficult for a none DIY'er or someone with no building/management experience. As I look around a number of building projects in my village the majority the reasonably good to bad ratio is about 50:50.
To work with individual tradesmen rather than one contractor requires a high degree of management ability, never mind the problem of finding them. Supply of materials in due time and quantity would be another problem for those without experience or the will to manage such.
You are gifted with such skills but I am writing for those without, as you and others like you will not need advice from me. I cannot see how an inexperienced person can do other than follow roughly what I have said, with a fair amount of common sense thrown in of course.