Just reading through the original post (and please bear in mind that I still play the classic edition from 1989 and not either of the two remakes 1990 or 2021 so there might be some conversion required for some of my suggestions - although I have heard that the latest remake does have cards like the original so there may be hope!) there appears to be a number of different but related issues, but most of what I can offer is prevention rather than a cure, which may not help you if it is already too late.
a) The remake editions removed the "cards limit supply" principle in the classic edition so that the adventurers (apart from the Wizard) can all be equipped with a helmet, battle axe, crossbow, plate armour giving them 4AD, 5DD & 3AD missile attacks whereas under the classic edition there was only one of most cards available, only one battle axe, one crossbow, one plate armour, one or two helmets (depending on 1st or 2nd edition).
b) The 1990 remake was nicknamed "the potion hoarder edition" early on and for good reason. Under the classic edition potion cards were kept until they were used and then they were discarded to be put back into the treasure deck in time for the next quest, or until the quest ended and then they were discarded. This second element proved unpopular with players, and for good reason, so the 1990 remake allowed you to keep them, but this lead to hoarding of potions which ruined the game. At my table, you keep a potion (and the potion card) until you use it and then it is discarded to be put back into the treasure deck for the next quest, if at end of the quest you have any unused potions then I make a note of these on the back of your charcater sheet, and you get those potion cards back at the start of the next game, so they are NOT included in the treasure deck for the next quest. This is a neat self-balancing mechanism that means that you cannot hoard, if there are only two healing potions in the deck, then there is a maximum of two healing potions in play at any time.
c) Limit the rate of gold acquisition: only rooms can be searched for treasure, each room can only be searched once
d) If a chest specifies a quest treasure then give that to the opening player, otherwise just draw a treasure card, if it is a potion, gold or something of value then give it to the player, if it is a trap then the chest is trapped (and empty), if Wandering Monster (or Nothing), then the chest is empty.
e) The Gathering Storm campaign (quests supplied with the basic box) is generally well-balanced in terms of advancement but there are couple of niggles: Borin's Armour is effectively a 850gc give-away, replace it with Borin's Helm (a helmet giving an extra defend dice AND and extra mind dice in defence against spells like Sleep) and the bounty rewards for Bastion of Chaos which give you gold for each monster killed (which effectively doubles the gold bonus) instead make that just a 200gc reward for killing all four Chaos Warriors. Finally the Stone Hunter and Fire Mage quests the quest rewards should be 100gc and 150gc for the party and NOT each.
f) The Gathering Storm campaign comes with sufficient gear for the adventurers to spend their gold on but the expansions particularly Kellar's Keep and Return of the Witch Lord don't come with any or at least not enough extra gear for the gold that can be acquired in those expansions, so create extra equipment cards, some ideas here
Gear Up!Note: As others have mentioned you can also ramp up the monsters, to increase the difficulty in later Quest Packs. There are many suggestions on how to do this in the forum, but I think that if you look at the full range of monsters in "Classic" HeroQuest there are a lot more than you think. It is easy to assume that there are just the 8 original monsters, the ones that you get monster cards for in the box (I assume for cost reasons they didn't include more Monster Cards in the expansions), but a little research into the expansions and a bit of lateral thinking and there are plenty more official ones before you even consider homebrew (I estimate around 24 monster types but that isn't an exact figure as I've ignored some unique ones like the Shape Changer and the Dwarf King's Skeletons, and some I have considered variants like Stone Chaos Warriors rather than their own type)
The Monster MashEDIT: Forgot to mention under (e) replacing Borin's Armour with Borin's Helm also enables you to get rid of the movement restriction associated with Plate armour