It's not OpenWrt limiting the TX power. The mt76 driver uses the max TX power listed in the wireless-regdb database https://wireless.wiki.kernel.org/en/developers/regulatory/wireless-regdb .
For USA UNII-1 bands, it's mentioned at (the url points to the db.txt in the wireless-regdb git tag master-2023-09-01) https://git.kernel.org/pub/scm/linux/kernel/git/sforshee/wireless-regdb.git/tree/db.txt?id=991b1ef696b7a034a5bf001cf31ab7735888c6e1#n1785
For Canada, it's mentioned at https://git.kernel.org/pub/scm/linux/kernel/git/sforshee/wireless-regdb.git/tree/db.txt?id=991b1ef696b7a034a5bf001cf31ab7735888c6e1#n348