I have an additional question about difficulty. Generally people say older games were harder and required more thinking. I'm all for that, but I can't quite imagine what that means and would be happy to hear a clarification.
Generally what I think tactics is comes down to three approaches.
One is "stealth": you use a rogue to scout out all areas and then you can actually plan the battle. Or use invisibility to sneak up on someone. Or even past someone, if they're too tough. Or lay traps.
Another approach is "disability", and it's mostly focused around mages. Spells like freeze, paralysis, charm, sleep, nightmare. An example would be DAO with a freeze spell + two spells that could shatter frozen creatures into pieces, fighters could shatter them if they landed a crit, and there were arrows that could shatter them, too. Force Field was handy to lock out the strongest opponent from the battle, as well as knocking opponents down.
The third one is "positioning": sometimes you can't win a battle if you just barge in and start bashing, and you have to place party members in different places. Possibly hiding some of them or making them hard to reach. Ideally combined with stealth to allow unhindered positioning before the battle.
If the game is still hard without requiring these approaches, then chances are it's a game that expects you to chug potions non-stop and win in a completely non-tactical Potion Way. (Or maybe it wants you to abuse AI with kiting)
So all that said, what about old games? Is there something different in their combat? Because to be honest, I can't imagine anything.