This is déjà vu all over again... You need to prove that you can keep his trees alive for a whole year before you repot anything else. Normally, it's very unusual to over water anything in the growing season, Pines excepted. Your description of his and your growing conditions sounds like his is more challenging. I suspect that you are repotting at the wrong time for your climate and situation. Problem is, I probably don't understand your micro-climate enough to micro-manage you from 8,000 miles away. Each species has an optimal schedule for major work, usually spread over a fairly wide period and many of us do things in seasons that others swear is wrong, but many of our climates are more or less forgiving for horticulture. I don't think of Beirut as having much cropland surrounding it, I think of pictures of Iraq where the farms grow dates, poppies and olives where the water is. If that is approximately right then you need to sneak in repotting at just the right time. That is when the plant is ready to spring into life in spring, where you have a more friendly period for growth long enough to mature a new canopy of leaves/needles before the weather gets too hot and unforgiving. The buds for the next year are matured through the following summer/hot season and are life or death for plants. Each part of the growing season has an important contribution, and none are optional.
Are you buying plants that you see in the local landscape? For the time being, you shouldn't try to grow in a pot those that you don't see growing in the landscape. Exotics, and anything in a pot take more effort and expertise. All of us do it, and we have plenty of ongoing losses because we don't or won't or can't give the precise care necessary. I lose exotics every year, and buy more. Been there, done that.
Buy the plants at any time, but do not do anything to them until you've had them for a whole year. For each plant you need to confidently know what dates a given plant starts to actively grow, that's when you do one major insult per year: either repot and root reduction, or major architecture reduction and wiring. The third option of leaf size reduction through defoliating is only possible to a very healthy tree at just the right time, too, so is a bridge too far for you until a few years down the line.