Yes, character progression comes to a relatively early finish in HQ, after 10-20 quests or so, as you said, beyond which the heroes are fully equipped except for novel artifacts. This is a known issue but there are even several solutions contained in the original material, e.g, the rust spell and quests that rob heroes of their gold. But these things need to be used with caution in order not to ruin the fun.
Also, if you play a well designed series of adventures, players can have a lot of fun just by going through the adventure and being part of the cool story without much character progression. Back in the 90s, we played the base game, Kellar’s Keep, Witchlord, and Ogre Hordes, ... expansions as written by the rules and had a lot of fun. If you play bad quest series, though, then the fun will be gone quickly anyway, of course.
Now, if you still want to get more out of your HQ game and are not afraid of using alternative rules, check out this thread:
https://forum.yeoldeinn.com/viewtopic.php?f=38&t=6622 ,
where you’ll find my own reworked HQ rules which I see as a more modern way of playing HQ. That set of rules, called Tactical Heroquest (THQ), is designed to solve a number of issues of HQ, including the poor character progression system. THQ heroes progess a bit more slowly when buying equipment and they may also increase their main attributes continuously. There are also regular ways (combat against fimirs and mummies) threatening with moderate setbacks in character development by destroying weapons and reducing attribute points. However, major hero improvements like acquired artifacts and a special legendary hero skill (gained later in the game) cannot be taken away, so my expectation is that the THQ rule system can maintain a decent progression of characters for about 50-80 quests or so, while also keeping the game balanced in all aspects. The retirement of heroes is anticipated to occur after ca. 80-100 quests played. (I am not yet that far.)
Finally, a small word of caution: Although THQ is not more complex than original HQ, it is not for everyone. It’s certainly not for casual players and really only for people who are open to more substantial rule changes. Some things are even simpler than in the original HQ, but the action happening on the game board is a different one. You won't guess what can happen there by only reading the rules. Try it out if you are interested, the mechanics are pretty coherent and are also playtested. There is also other good stuff included and I am still in the process of extending this further without overloading it with too much extras.