FWIW, I'm a seasoned amateur bike mechanic and I can fully tear down and overhaul any part of a bike besides a derailleur that's riveted together. There were circumstances that lead me to acquire a new GMC Denali from Walmart, with my eyes wide-open to the widely reported quality and durability issues. When I assembled the bike and dialed in all the adjustments, it worked as well as could be expected, i.e., more than adequately, but not by much. However, over time, one issue after another has arisen, fully as I expected and was prepared to deal with. For instance, I decided proactively to service the bottom bracket, very early in its life, and found the notorious paucity of grease, and unwanted metal shavings in the bearings. The headset brinnelled (not sure if false or actual) very early, and I replaced it with a surplus vintage Japanese headset. Recently, I wanted to remove the freewheel for cleaning, but with much less torque than I would consider to be normal for freewheel removal, the splines in the freewheel body "stripped", i.e., not that they sheared off, but they rounded off and expanded diametrally, allowing the freewheel removal tool to spin uselessly inside the freewheel body.
Memory fades, but I do believe I overhauled the wheel bearings as well, pre-emptively, and found on teardown that they had just the bare minimum of grease (if that), which I remedied, of course.
Hence, I have already acquired a high quality vintage wheelset at low cost, to replace the entire, depressingly cheap original wheelset (actually, the low-end Shimano rims are fine, but the pressed steel hubs are nasty). The runout on the freewheel threads is appalling (judging by the freewheel wobble as the wheel rotates), the worst I've seen (it certainly doesn't help with dialling in the RD adjustments). The RD is a monstrosity of plastic and pressed steel; it too will be upgraded with a modest but decently made, used Shimano Alvio, acquired at low cost, of course.
The brakes were awful from the beginning, but I had a nice surplus vintage brakeset that made a nice upgrade.
I'm not writing this to complain about my experience, since I was fully prepared for exactly such issues when I made the purchase, and had a plethora of vintage parts on hand with which to make the upgrades at little or no cost; only to warn that if you intend to put a lot of miles on a low priced, big box store bike, issues will arise, and if you can't deal with them yourself, it will tend to be costly, much more costly than a decent bike, in the long run, is my guess.

IMO the best bang for your buck is a used bike of the right type that is already in good tune. Of course this is hard to establish if you are not experienced in evaluating a bike, but in that case you need a buddy to do this for you, or else purchase a used bike from a reputable business or seller.

I should mention that I enjoy working on bikes, whether maintaining well made components, or replacing lousy ones with better ones, but I detest and try to avoid having to do "roadside" maintenance, as I'm sure anyone does. That is probably the biggest argument for making sure your bike is adequately spec'd and maintained for the use you expect to get out of it.