For short rests, I made them 5 minutes long, and gave 2 short rests per long rests. I do like the simplicity and scaling of keying it off proficiency bonus, however. Would have been a great mechanic to have in the original PHB.Not sure what you did, but the new paradigm appears to be a number of uses equal to your proficiency bonus per long rest. You would need to adjust for some classes. For example, I might give the battlemaster 2x profiencey bonus # of maneuvers per long rest.
I don't know that I agree with that. I've made a lot of custom monsters throughout 4e and 5e and am fairly familiar with both. I think we sometimes forget 4e monsters went through a number of redesigns before the hit their sweet spot in the MM3 and after (really the essentials line of monster books were the best IMO). We also forget there were a lot of fairly boring monsters in 4e too. Finally, there are actually a lot of interesting monsters in 5e (even more so after the MM). In fact, I would agree the legendary monsters are more interesting than their 4e equivalent 80% of the time.
Regarding 4E monsters, I'll take your word for it. I've not played 4E. But I have the books. Looking at frost giants, for example, the 4E versions are much more varied and evocative than what 5E offers. In 5E, spellcasting monsters can be painful...especially those built on the warlock chassis. I really hate having to calculate mid-battle how many eldritch blasts a monster gets! I think this is part of why "The Monsters Know What They're Doing" has been so successful...because there's often an optimal way 5E monsters are meant to be played, but for complex monsters that way is obscured in the stat block.