Yep. I've had some players buy the module after I ran it, write long emails saying "that's not the way the module said to handle the tactics" or "that's not the way some streamer" does it. While I try to correct this behavior, it's still very frustrating.
Definitive answers would help my personality type and give me a better leg to stand on against these players.
Oh that indeed sounds painful. It seems to me like you need a session 0 to address these issues because you certainly do not want to continue down this route. I experienced some (the rules thing) during the 3.5 era, it is not a healthy space to be in as a DM and partly was my fault because I never properly acquainted myself with the rule set. I'm much happier with 5e, compared to 3.5 or pathfinder, it is rules light with much more DM empowerment.
As for the issues you have experienced, I can sympathise, but I have found more enjoyment with the system after making it my own. The toolsets that exist within the DMG as well as online allow you to sculpt a better game.
As for the combat issues you have described. Think less in terms of hit points, x movement and sword swinging and rather view the scene organically.
Let the kobold throw dirt in the eyes of the PCs hoping to escape, and have the PC makes a dex save or losing their reaction or bonus action on their next turn.
Have the ogre tear the holy symbol from the cleric's hand or neck to show how weak and pathetic the PC's god is.
Make the earth elemental earthglide under the PCs before slamming them from underneath, sacrifice x damage off the attack to make the PCs make a save or be knocked prone or sacrifice the entire multiattack to make x area underneath difficult terrain.
Have the dragon grapple a PC, lift them into the air at their maximum flight distance in a round and then drop them (= falling damage, prone, likely disarmed, shield scattered - at the very minimum)
Better yet have the dragon grapple (lift) them with their Tail Attack (Legendary Action) then double move (160 feet) and drop for free.
There are wonderfully innovative ways to handle combat, do not be hampered by the stock standard stat block as reflected in the MM. Use the narrative provided for each monster as inspiration on how they behave. Use weather conditions and terrain as possible obstacles in combat. So the beholder missed his Disintegrate spell, maybe his beam hit a pillar causing structural damage, falling debris (damage) and difficult terrain (additional complication for the PCs).
Remember every miss is an opportunity for some great storytelling. Don't squander it.
