Try:
1. Zcova (Kota Damansara). They used to do promotion for 1 carat cushion cut. They have a GIA GG to help you pick. Offers customisation service. Has a physical premise to meet up.
2. The Gem Tribe (Cheras). They do some nice settings as far as I am concerned. They have a GIA GG to help you. Offers customisation service. Have a physical premise to meet up.
3. iDo Jewellery (Bukit Jalil). They do some nice settings as well. The owner doesnt seem to be a full GIA GG, but she has vast experience. Offers customisation service. No physical premise so usually meet up at cafe around that area.
4. Lavin New York (Bangsar South). Family run business helmed by 4 GIA GG to help you. Offers customisation service. Has a physical premise to meet up. Usually offers pretty good prices.
5. Eumayco (Bangsar South). Long time business. Not sure about the expertise of their people, but they have been in this industry for very long time. Offers customisation service. Has a physical premise to meet up.
6. House of Diamonds (Publika). Recently one of the person that ask me actually bought engagement ring from them. As far as I can see, prices are pretty fair. One of the owners is a GIA GG. Has a physical premise to meet up.
7. W Jeweller (Subang). I have been following their page for a while. Not sure about expertise, but they seem to have garnered huge experience over the years as a family run business. Offers customisation service. Has a physical premise to meet up.
Since you already have your own requirements, try to survey a few of them above. One thing I have to say that while a lot of jewellers (not to say anyone in particular) has a GIA graduate, in actual fact, they may probably differ in view on what is the best stone.
Also, most of them should be getting stones from suppliers and not stones custom cut to their specifications, which is an expensive thing to do, hence, the chance to get a nice stone compared to those custom ones are lower. But still, not impossible. Overall, my advice is try to approach more and see who gives you the best confidence and makes you more comfortable. Try to survey more than the listed ones above. You can even walk-in to chained stores and see as well.
And one more thing... Cushion cut has a few types I believe (you can Google it). So you may want to see what exactly you want. There is even Hearts and Arrows cushion!
That's all I got to share. Good luck.
Thank you for you input ! I’ll definitely approach more stores. What do you think of my preferred requirement above ? Those requirements are based on my research and reading. Anything in your view that I need to adjust/change?