Another reason not to return a 404 in that case is that chances there will be monitor tooling in place that will treat a 404 as an "error" that will show up in your alerting, but would not be ideal; it will just be noise.
In my 40s. I feel that way too sometimes. My advice: try new things! I just started motorcycling; signed up for MSF course, got a used bike and it’s awesome. I play beer league hockey and that one night a week playing and being with the boys is precious (and theuropeutic…in a house with only daughters and wife lol). Snowboard in the winter…got my kids into skiing…golf in summer…LeetCode for fun to keep my mind/coding sharp. You get the idea. My day job (Android coding) is kinda mundane and is really just for the paycheck…and so my personal philosophy of life is to keep trying new things, diverse things, not being afraid of new things, and just going for it. I’m never bored and I don’t sweat things like “legacy” and whatnot. Enjoy life!
I'm in my 40s doing the leetcode grind, but weird thing is, I'm actually ENJOYING it. I get a little high when I solve a problem, and I feel good about re-learning / re-discovering data structures and algorithms that I just don't use on a day-to-day basis. I paid for the premium version and enjoy reading the solutions and maybe learning a new trick or algorithm technique or whatnot. And who knows, maybe something that I learn when grinding on some leetcode problem will actually be useful in my day job? Thinking about it, if I ever do land a MAANG job, I don't think I'll stop doing leetcode problems. Sure, I won't go crazy with it like I'm cramming for mid-terms or something, but I'll still hack on them for the pure joy of problem solving, for the sake of problem solving.
I guess where I'm going with this is, maybe change your thinking about leetcode? Don't think of it as a necessary evil in order to land a high paying job that you dread doing each night; look at it as a fun little hobby, with the nice side-effect that you're keeping your data structure knowledge, algorithms and general problem solving skills sharp.