Quality reduction happening on sending videos does not happen on your phone, it happens on the carrier once the picture or video leaves your phone so the only thing you can do about it is avoid sending it through your carrier.
You can send them through Google Photos instead.
It could also depend on your carrier network and the receiving phone. If you are on the same network, and the recipient's phone is capable of receiving RCS, you should both activate that to send uncompressed messages via text.