Where is MT6000's SDK?

I'm not seeing MT6000 in GitHub - gl-inet/sdk: OpenWRT SDK for GL.iNet devices

Where and how can i get its SDK?

I'm trying to build a custom package to use with MT6000. Thanks.

MT6000 is supported by OpenWrt 23.05 and newer.
Just use the OpenWrt toolchain or SDK to compile packages.

I notice the stock FW is on 21.x; wouldn't that require a 21.x based SDK?

Actually I managed to just build a random target for mt7622 and that ipk did work for my router (with stock FW). (seems like mt7622 is the only mediatek chip type in upstream 21.x set to 64-bit; the stock FW looks like 64-bit)