You're big enough to be in the realm that bikes engineered around the median rider are going to be problematic in ways the Lilliputians cannot understand.
The car analogy is close, but flawed in that cars require bespoke hardware to run - bikes are a lot more standard fare, and in a lot of cases are comparing which manufacturer happened to turn out a model of those generic Freightliner/Dart/Merc work vans. They're interchangeable as far as parts, and what matters most is the spec of the components on the frame, since junk parts on it determine the performance more than the underlying frame. Worse yet, bike manufacturers get much better deals on groupsets, which means it actually is more expensive to build a bike than just accept whatever peanut-counter approved spec a brand is shipping out with that frame, so building yourself only saves money if your intent was to swap out a lot of those parts anyway.
The real issue is that there's a bigger difference between cheap (both cost and build quality/parts quality) bikes and one worth putting more money into; much larger than the difference between a civic and a Wrangler. Think of it more like computers of comparable cost range - you have to get above a price floor (usually around $800 for desktops (hardtails) and $1400 for laptops (full suspension) before you end up with better interchangeable parts at all, and so many models are just stripped out versions with cheaper filler parts than what that platform was intended to run on (go to NewEgg, look at an entire line of identical looking laptops: the cheapest one to somebody who doesn't understand the underlying spec and performance will be mystified to the cost difference, while somebody who knows what each part does will see why that same case with different parts is total crap and the mid-range and above flavors are the only ones worth buying).
So you're right about LBS trying to sell people on newer bikes, mostly because those are easier to service for them. You'll honestly have to think about bikes like buying computer hardware - research a crapton, and go in knowing more than the over the counter sales jockeys who may or may not care about MTB riding themselves.
Buying used bikes is just as bad as computer hardware again - unless you know what you're looking for, the tendency is just to inherit somebody else's problematic equipment, which can be even more costly.
Finally, you do need to understand which parts make a performance difference FOR YOU, and which don't. For some it's the wheels/tires they notice most, others it's the drivetrain/pedals, and a lot of people care probably too much about suspension travel and feel. For me it's brakes/tires, but I know that's not the norm. It's a lot more about understanding which parts you care about. Analogy again - Jeeps are really different because anybody who actually leaves pavement with theirs want something different, which is why you see huge tired rock-crawlers, or more streetable versions with some engine mods; or you'll even see very capable jeeps where the off-road capability isn't the direct immediate goal. The difference is that Jeeps have a lot more DIY improvements, and that market is larger and generally smarter when it comes to buying parts, better mountain bikes have the problem of a very large number of uneducated buyers who want something which looks awesome, but may not get used enough to expose just how crappy a lot of the parts are, whereas a Jeep will get daily driven, and then taken off road and wheeled until something breaks or a winch is required - so crap parts get weeded out. The higher end bikes tend to be bought by people that are ironically more casual bikers, or are hardcore bikers in the sense that they're high earning professionals who are very passionate about bikes and spend their free time riding them, but that most of the true enthusiasts can only afford mid-grade hardware.
Why this site tends to be an enabler-pile is that with the right knowledge, a 5-10 year old bike can be super competitive with new equipment, but it takes a lot of the right information to make that work. The issue is that mountain biking over the last 8 years has received a LOT of really good hand-me-down tech on the suspension, brakes, and material science front that has only partially tricked down to the reasonable price range (Carbon fibre frames/wheelsets are absurdly expensive, but Shimano Deore Hydro brakes are a phenomenal value; the RockShox Pike RCT3/Charger with Monarch R+ is a great shock system, but the real value is still in the simplest Fox34 and lower end Revelation with the basic FoxCTD or Monarch). To make that worse, the bike industry is exceptionally stupid about making certain specifications obsolete - with computer stuff there are performance reasons, on the bike side a ton of dumb standards are regularly adopted, and just as quickly abandoned.
The prices are equally absurd - in a lot of the component specs, knowing what I know now I can pick out the 1-3 items where they deliberately took an inferior part in order to round the price out to exactly $799 or something, instead of making it an $817 bike that was substantially better. Worse is when they spec on just ONE component from a higher component tier, especially if it's one that independently can't improve the riding experience (e.g. an X9 shifter on a bike with an X7 derailleur, and X5/Deore parts everywhere else does not make it an X9 level bike, but they will advertise it as such; especially if a bike would otherwise be $1270, they'll spec on one part that costs them $15 more to make it $1299). It can be pretty bad at times.
So, as much as it sounds like selling both would be a 'bad' idea, it's not. For a hardtail XC focused bike, the worthwhile options DO open up right at where your budget would be after selling both. If you can get to something with tubeless ready wheels, a solo air fork (The RockShox Sector ideally, though SunTour's Raidon has improved a ton), hydraulic brakes (mechanical disk suck for anybody over 180lb), and a decent frame then you're so tremendously better off. Anything less is like getting a Camaro body with a Geo Metro drivetrain, there's only so much hot rodding you can do, and you're still money behind just starting off with the V8 version that comes with better brakes/suspension/interior; same idea.
