To be honest, the game now has a lot more mechanics and variability for tactical setup against the enemy packs. Even when the EA just started there were limitless possibilities to setup your party in a way to defeat even the strongest of orcs. There is no “one right” party setup to win it all. There is just too much variety and it is growing with every content patch.
If you have a firm understanding what the role of each individual party member in your setup is, and you lvl their stats and perks accordingly, equip them with the most beneficial items for their tasks, then you should not have any problems. There are no artificial limitations, you can make a two archer party where one of them is with rally while the other with focus and high fati turning into an SMG every other round. Also many new players fall into the trap thinking that there are only 3 useful stats ignoring all others. For a character with high starting hp sometimes it is more beneficial to get the colossus and some lvl ups in hp than to get anything else. A character with high starting resolve is ideal for the captain perk and leveling resolve has obvious benefits. You mentioned goblins, do you have even one character with high initiative? So he can move first, equipped in more light armor and perks to close the gap to the enemy archers fast without dying? Ever tried to make something like that? Do experiment with build.
Equipment is a vital part of the game both as individual character setup and the party as a whole. It does seem at first that just equipping all in heaviest armor and strongest weapons lets you roll over everything, but that is simply not true. As you obviously noticed with your current setup. It might have worked with bandits, even skeletons fairly well, but you should have had problems with big undead groups and obviously is countered with fast paced goblins and the orcish horde. Goblins easily evade your slow guys, further slowing them with bolas and nets. And orcs, well they are the strongest enemies in game, you are a mere human and can not overpower them in mere strenght.
You have a wast of weapon and armor variety. You can use blunt weapons to stun, and it synergises well with the push the advantage perk. You can use shield bashes to change the enemy line, hooks to pull and pikes to push them around. You have nets, dogs, and other variables to controll the enemy. If you can’t take em all at the same time, slow down part of them and deal with the remainings. Combine the attributes, perks and equipment on individual members. Even without perks all the sellswords have two additional slots, you can put there throwing weapons, you can put there range wepons, shields and secoundary weapons. Just remember it does drain the fati of the mercenaries when you put stuff in there.
Also there are lots of different backgrounds. With a full party of low end backgrounds like beggars monks and other non combat oriented professions and accordingly lower starting stat characters you will have a hell of a harder time getting by than with a full party of high end fighting professions. It is just as much a management game as it is a tactical combat game. Don’t be shy to get rid of weak brothers replacing them with new strong ones. You are the boss, they are your employes, your tools to survive in this world. That is why the constant loading of the game is imo a very bad choise. The flow of recruits should be constant so the weak ones will be removed by the game itself.
If you pay close attention to how the AI plays you can see that the factions play quite differently. Some have even multiple playstyles depending on situation. It is a tactical game, if what you are doing does not work, do try different approach. No need to get frustrated when you can’t seem to get a grip on what is wrong and why you getting hammered by the enemy. You can always try new stuff, change things, and ask on the forums. Here are many people who will share their experience thoughts and advices on the game. Just by reading your thread you can see how differently people play this game, and that it is working for them. Do not approach Battle Brothers as the nowadays common “only one way of play to win” game, the more generic you go the less fun you will have and more frustrated you get.
You are no heroes, just a bunch of mere humans in a harsh unjust world full of scary things that are eager to eat your flesh and devour your soul.