creme patissiere too solid
The starch was superfluous. Traditional creme patisserie is made without it. Some people do make custards which combine starch and yolks, but they are actually more difficult to work with. Also, from your ratios, the starch alone (without the yolks) would suffice to make the shape-holding pudding you described. 

You can repeat the whole cream without any starch, and as long as you use the right temperatures, it will give you a nice pastry cream. The UHT and the milkfat content are not a problem, you can do this with anything from pure water to conditor cream, and while you will get a large difference in texture between these two extremes, it will be a difference in "butteriness", you won't get a firm result.