Unless you have RCS in samsung messages it will compress the video. Try sending in Google messages, provided the other party has it also. You could also send in Facebook messenger.
quality gets lowered if you send it through txt message.
if you want to keep original quality, send link share, or use 3rd part messaging app like whatsapp
if you have chat settings then you need to enable it. As long as you connect with someone else using RCS you will be able to send large pics and videos. If it says chat message where you type it means the person you are messaging has it also. If it says text message it means they don't. It's kinda like the blue and green chat bubbles on iphones.
Part of the issue might be that the two people I send video and pictures to most often are iPhone users. I have every advanced feature available on this phone turned on, I believe. Aside from switching to Google Messages when I get home so I can continue to text from my computer, Samsung Messages is on chat. I really wish they'd get a web version of messages going. I really dislike switching apps and am not a huge fan of Google Messages
With Iphone users you would need to use Facebook messenger or whatsapp on both ends to send and receive large videos uncompressed. Maybe one day they will standardize messenger across all platforms.