A few thing to consider, although CNMG and WNMG have more edges, does it really make a difference in the long run at the hobbyist level? A CCMT insert can last me 3-6 months and I pick them up for around 2-6 dollars each. As Mikey outlined, the negative rake tool holders in general do not perform as well in small lathes with less Hp. There is also a wide range of inserts with different coatings and edge profiles, so although the holder may be negative or neutral rake the insert edge can be highly positive. In addition, usually the larger the insert holder, the wider availability of inserts, they are much more plentiful and less expensive on different auction sites. Do not buy generic no-name inserts like Cobra, etc., they are cheap and they do not last. I buy name brand inserts such as Kennametal, Iscar, Korloy, etc. and they work much better. But you need to know a bit about the insert types, coatings and profiles. The cutting speeds are more for production level machines, I routinely run my inserts at 1/4-1/2 the rated SFM with great finishes and no cutting issues. Part of this is also the rigidity of your lathe/holder. CCMT insert holder is considered a neutral holder (second letter defines the holder rake), WNMG insert holder is negative.
My everyday insert for cutting steel is a CCMT, and I use both CCMT and CCGT inserts. For softer materials I use a WNMG holder with highly polished uncoated carbide inserts with a sharp positive edge. They cut great, I usually change the cutting edge every 2-3 months so an insert lasts a long time. If one breaks, takes no time to replace it and I do not need to reset my tool holder position or DRO position setting. I have a wide range of other insert holders for different profiles, all my other insert holders are neutral "C" type holders which seem to work well on the lower power lathes. I use different inserts depending on the material, the depth of cut and the type of cut (heavy, light, interrupted), but usually for my CCMT I usually use a single insert type (from Kennametal) that has a wide cutting performance in most materials. I have attached some notes I put together.
The G4003G should be able to take a BXA holder, so 5/8" tool holders would be more optimal if it fits. If you had a 13 or 14" swing you could use 3/4" holders which can give a bit more rigidity providing your lathe is also more rigid.
On the QCTP holders you should have enough adjustment for the full adjustment height on the tool post, it might be the brand you have. Otherwise make your own threaded adjustment posts that are taller.