So far not one person seems to understand what you wrote rather than what you wrote being incorrect in any way, shape or form.
If the spiders were too difficult then go back later once you have gained a level. That fight is difficult but there are several things which can be done to make it easier.
You can talk Yurgir into killing himself as well as his minions. There are lots of vids on YT of people defeating Yurgir and his minions. Is this the fight you were referring to when you said "dozens of opponents"?
Average enemies do not "have a bunch of resistances". They may have a few but they are only resistances not immunities.
As has already been said, what you get at level up is mostly down to 5e rules, including the number of HP you get being dictated by the class.
"I had two figters, a mage and a rouge in my team. All of them had poor features at leveling up"
By level 5 all your party will have chosen a subclass and a feat or ability score increase. Your mage will have several more spells of various levels; the fighters and the rogue will have received more actions and passives; the rogue's sneak attack damage will have increased twice; and the two fighters each got an extra attack.
Unless I am missing something, at no level (1-12) does any of those classes receive only HP increases.
https://bg3.wiki/wiki/Fighterhttps://bg3.wiki/wiki/Wizardhttps://bg3.wiki/wiki/Rogue
