except those rules they present for doing it are spmewhere between half baked & destructive at best.They are saying that you can change the length of rests as you feel appropriate, and that the DM shouldn't include content (classes, races, spells, or especially feats) in their world that they don't explicitly choose. While that doesn't give much to go by, it still provides strong contrast to 4E, where "everything was core" and mucking about with sub-systems would cause the whole thing to collapse like a house of cards.
What it will do is demolish interclass balance between long rest & short rest classes making short rest classes much better & long rest ones feeling a bit crippled. On top of that you wind up breaking many spells & class abilities to the point where they are useless or not worth the new now much higher cost. Eventually you might find what you think is a decent balance of how often you should allow long/short rests there is the supernova of broken that comes with pretty much every magic item generating max charges each long rest & getting quite a few overnight during the short rests known as "sleeping as living beings need to do"... That wand of magic missile, fireball, or whatever can now be used one or more times every encounter with almost no need to bother holding back. Stay away from this nightmare unless you are looking for a game that feels like a few T1000 terminators mowing down hapless civilians.
unless the goal is pointless hoops or overpowered to a broken degree it’s probably not going to come from adding either of the noted rules, the related ones, or some combination because so much of 5e is written to fight any attempts at dragging it kicking & screaming outside a very narrow scope of valid gameplay.
You keep bringing up how you can change the rest durations & how there are rules in the dmg, but the fact that those rules are so half baked that it's reasonable to ask if they were deliberately written in such a fashion that a gm could not use them to force what someone decided was nothing but badwrongfun.