Just why send bridge crew. Those crew are important and mean to stay on the ship. Unless the mission required someone on the top like first-mate, you should send your boarding team or land team on mission.
Most of the eps, I see a dangerous mission with 2-5 bridge crew member. Sometime it is include both captain and first mate.
Sometime, it is fighting sorties, a first mate said: I need volunteer who is trained in close combat. And multiple bridge crew go with him. The next scene is the crews start shoting handgun in some dangerous place, maybe they also teleport inside enermy ship (boarding action), and sometime is fighting kungfu.
Why not send a sergeant and handful of soldiers ? The ship is quite big but there are no spare personel to send ?
To keep the number of characters in the core cast to a reasonable size.
You can still see the remnants of trying to address the “realism” issue with things like Riker existing at all. Writing in an XO was supposed to divide responsibilities; keep the captain on the ship to make choices and put the XO on away missions to karate chop Klingons. However if that had been stuck to rigidly, Picard would never be written into many exciting situations.
MACOs on ENT should have logically made Malcolm redundant, but the show kept finding things for him to do.
Not really. It would relieve Malcolm of his security role, but he was still tactical officer. A lot of naval ships had marines on board to serve security roles while weapons maintenance and operation would be performed by a different group of naval officers and seamen.