This was my other solution, the lightest quill stem you can buy today - from Aliexpress.
174 grams for a 31.8mm H/bar and 100mm long quill stem is among the lightest ever produced.
The stem is an UNO stem with ti bolts which weighs 95 grams, it is fitted to a quill adapter which is designed for kids' balance bikes but sold online as a normal quill adapter. The standard product only weighs 93 grams but with an M8 x 100mm ti bolt it weighs 80 grams.
I also bought a 31.8-26 mm handlebar shim made of UD carbon, I tailored that to fit the stem with a Dremel which adds about 9 grams.
The adapter is so short it should only be used for slammed stems, there is only 60 mm that can be instered into the fork steerer.
To be honest I'd be a bit worried to fit the set up, it's probably strong enough but I have doubts.