Rate limiting by MAC address only seems to work for uploads.
There must be something wrong with the creation of the nft-qos-mac download chain.
I doubt that a destination MAC address can be used (successfully) in a chain on the prerouting hook of an inet table, but I could be wrong.
If you think it's worth the trouble, you could create a table of bridge family.
opkg update; opkg install kmod-nft-bridge
nft add table bridge nft-qos-mac
nft add chain bridge nft-qos-mac download '{type filter hook postrouting priority 0; policy accept; }'
nft insert rule bridge nft-qos-mac download ether daddr 3c:7c:3f:23:5c:50 limit rate over 6250 kbytes/second counter drop