A lot of the sequels can get pretty damn hard because they sort of built them with the idea that you'd be bringing in your previous party. Although the previous party ends up being your downfall in the last game, Pools of Darkness. Because SSI stupidly (in my opinion) decided/were forced to follow those AD&D rules to the letter; the simple fact that you have the same characters through 4 games means by the 4th (really the end of the 3rd) you are now unable to train any non human character, besides a thief. In fact, if you want the best chance to win them all (the 4th one can be a bitch) you have to eliminate a huge part of the role playing aspect. If you're not playing with all human characters and you're not starting from the very first game; you're in for a fight.
Now; there are skill modes that I believe were introduced in Curse. Defaults in the middle at Ace, but there are harder and much easier levels. I think they attempted to also give you consequences for choosing a rookie level but not really... I think you face more people in "random" battles on easier modes, but in fact it's a lot easier... But you also get less experience in those easier modes... By the last game the rookie mode might byte you in the butt as well. But that is an option... I just stick with the defaults.
I always thought Curse was the easiest and shortest of all the gold box games... hahha... You've got much more to experience, young one.
You do have some control over your environment with the combat. Those walls and rooms in the combat section are not randomly generated. If you mapped the combat area you'll find it corresponded perfectly with the real 1st person map. The way you're facing when you go into a battle, how far away you are, the front of the back door... If you're in a corner in 1st person view then you'll be in a corner in battle and vice-versa. There are indeed times you can use this knowledge to your advantage.
The first couple games are all about the sleep spell and stinking cloud, maybe a magic missile or a hold person or two. The 3rd and 4th games bring in the big guns, Fireball and lightning bolt, hold monsters... Positioning is ever more important in those battles because a well placed fireball can be all you need... But they're both in same level for spells so you can't just load up on both fireballs and lightning bolts. Both very useful but for very different reasons.
Your party is confusing to me; but I'm often amazed at the combinations people do and end up winning. My default is a fighter, Paladin (as soon as they let you have one, which means I do have to get rid of a character at some point), multi class fighter/mage/thief, multi-class fighter/mage, cleric, and mage. As far as I'm concerned a thief is a must have, you really end up getting hurt quite a bit when disarming traps without one. But - You're right, they are rather useless, as their own class... Multi-class is where to go with the thief. Train him up as a fighter and a mage, as well as a thief, and you suddenly have a very valuable party member. The backstabbing is INCREDIBLY useful once you've figured out how it works... and it's okay, I don't blame you if you haven't figured that out... It's a pain in the butt... But once you know how to backstab with a thief you'll get a guaranteed hit with double the damage anyone else will do.
Keep me informed on the freezing issues. And if you do use my download link and still have issues, be sure to tell me... Because "if" I indeed did face that problem at some point.... I'm just not the kind of person that would have tolerated it... On rare occasions I recall digging into my original emulation files from back in the 90's to get games to work.... Normally I hate using those games because of cracks or whatever... But I have noticed the stuff I got back in the 90's tends to not show up these days. Back then literally one person cared enough to figure out how to turn his "real" disks into adfs, and EVERYBODY using emulators used this one dude's disks. With TOSEC now, there are now several versions of most games out there, even if most sites will only give you one of them. But I will vouch that TOSEC does not ever seem to carry "random dude's disks" from the 1990's. Most of the time this is a good thing, but I have on occasion gone back to my archives to dig up a game or two, and that ends up being the one that works. - *If I came across this freezing issue, I would have stomped on it. The more I hear your struggles, the more I kind of wonder if just perhaps I did experience these struggles with the "newer" adfs I found... But that I mostly forgot about it because I would have went back to my original 90's downloads and that ended up working. Hmmm.... I'll give a full report whenever I next dive into Curse for you... But if the issues goes on, I know my disks, wherever I got them from, should work.... So I'll be happy to grab them for you if need be.