But while I do think appreciating what you have is part of how to avoid the hedonic treadmill, I don't think it's a matter of learning to be happy with a routine.
I've found it possible to make a conscious effort to avoid hedonic adapation, and enjoy novel things without allowing them to become a new baseline. If you can maintain your expectations at the same level, while improving your actual circumstances noticeably above that level, you can maintain a higher level of enjoyment of your life.
Single, full-time working people, especially in tech, start to unlock the ability to travel abroad, maybe even one of those big two-three week Life Changing things to somewhere exotic, but only once in a lifetime or once every few years.
Middle / upper-middle class incomes eventually get to a point of wealth and freedom where they can take vacations abroad multiple times a year.
And I think you need to be even above that level, where you don't have to worry about your base income or mortgages or whatever, where you can have this lifestyle where you can have novel things and experiences all the time. And even then there's a risk people get used to it.
In general, these are just different goals in life. If you want a house and kids more than you want the freedom to travel, that's cool and pretty normal. There's no stone tablet handed down from the skies that says that has to be the way it is, though.
When I was 23 I saved a few grand up and went hitchhiking around Europe on a budget of something like < 500 eur a month.
I didn't care about my base income and still don't. I think most people accidentally structure their life so that the 9-5 becomes necessary (e.g. if you rent a room in an expensive city, or a flat or house, now you need to pay for that).
The income is a means to achieve the things that you want. If you can't X because Y, and you want to X, then give up Y, not X, find a different way to do X.
I have a mortgage now, but I didn't bother with it for the longest time for precisely the reason you'd described. It locks you down. I waited until I was sure that it _wouldn't_ lock me down because I could really afford it (i.e. it's not 50% of my income from a full time job).
If I'm being charitable, I had the advantage that I could move my belongings etc in to my parents' house if I wanted to. They're pretty poor, probably in the bottom 20% in the UK, so that's not some fantastic privilege.
If you have fuckup parents (not just poor but failed in some way), then yeah, this becomes a lot harder, and I'm sorry about that.
Beyond that, if you couldn't find a way, then it's just not your priority, ya know?
Russian blokes from mining towns can afford to travel third class on the Trans Siberian, what's your excuse?
And I'm sure we'd have fun too!
The opening of "It's A Wonderful Life" addresses this directly. It was undeniably George Bailey's priority, but even the "golden billion" aren't free from obligation(s).