As a DM, I find creatures that players are familiar with, if they use good tactics, can surprise the hell out of a party that was expecting them to be pushovers. Also, don't worry too much about non-boss fights. For bosses and even some non bosses I consider the MM to be the basic commoner version of a monster. Just like a commoner stat block doesn't mean every human is 10 in all stats, why should all beholders be exactly the same hp or have the same stats. The MM is just the base. There could definitely be a named beholder with magic resistance and legendary resistances and a couple extra options for legendary actions that has 8 extra hit dice. There can definitely be a vampire lord that can cast spells as a 17th level wizard and wears an amulet that gives them resistance to radiant damage. That troll that regenerates unless it is hit with fire can definitely regenerate twice as much normally or if hit with fire just regenerates the normal amount. That tarrasque might need a wish spell to permanently prevent it from regenerating back to life like it did in 2e. In fact, look at the MMs from earlier editions and use them for inspiration. Add class levels to creatures. I took a CR 5 hill giant and turned it into a CR 12 monster by giving it 3-4 levels in barbarian.