Beginner gm seeking advice before my sanity goes out the window

Overly dramatic title aside, here is the situation.

First off a little kind of disclaimer. So far everyone has fun but at this point I realized, that most encounter, even bosses don´t impose any real threat to the party and I think it´s mostly my fault, so I try to find a way to ballance things out a bit.

I am not trying to tpk them or kill single charakters on pupose. I just want a bit more challenge. 

Currently running the crooked moon as my first campaing. because of this, I decided to stay as close as possible to the books descriptions. 

The Party consists of six players. clearly my first mistake (but sessions take place when at least 4 are available due to the Campaing being intended for a party of four and it shows in regards of how encounters play out)

Characters in the party are: bard, druid, barbarian, cleric, paladin, rogue. no multiclasses so far.

We are halfway throug the story so far and it get´s hard to even put a scratch on them

well... this party has a quite strong synergy (also the Campaign throws magic items left and right on them). Experiment barbarian uses his maddening fumes trait to force anemies to do a wis save or have disadvantage on every target except him. Twillight cleric has a AC of 24 due to a plate armor he stole (whoever thinks to do the campaing, looking back i strongly advise to change the part with the three knights in the crimson monastery stables to the knights wearing their armor, so you can claim that the armor got destroyed in battle...), the druids´s sanctum of the sheperd, the shield of Marius Renathyr (that grants him a whooping +4 AC) and the druids wicker effigy granting +1. also whenever the unlikely event happens that he get´s hit, he uses the shields reaction to just deny the hit (most common enemies have not that good + to hit.. so they barely ever hit him aside from crits oder close hits) he also has twillight sanctury active in most encounters, so the largest part of damage dealt to any partymember get´s blocked or undone by regained temp HP. 

difference shows whenever this player is not present in a session

Now I guess most people would advise to use abilities with saves, bringing me to the next part of the synergy. The party allways crowding around the paladin, grants them +5 on their saves. prime target for a fireball (or similar aoe), right? 

yeah well, none of the casters so far has fireball.. or dispell magic or counterspell. All the aoe stuff aside from insect swarm on a higher level in a location that would be highly disadvantageus or the caster, is recharge 5-6. Also damage get´s undone by either twillight sanctuary or the tons of healing at their disposal. simply throwing more minons at them? yeah, hypnotic pattern or spirit guardians... it´s kinda trivializing the encounters and everyything I try to ballance things out seem to only lengthen encounters.

Also whenever there is a situation where during travel or that´s not happening on a battlemap and any kind of saving throws is needed, the paladin claims to keep everyone in 10 ft. reach around him. (absolutely trivialized the Hartsblight forest chapert so far, where a dc 10 Con save is needed whenerver the party travels to the next location or they gain a stacking debuff) 

so that brings me to the question, what would be your advise on how to ballance things a bit towards tha party size and situation? 

Giving more HP to everything just makes encounters go longer so far. 

More enemies seems not to be very effective so far. Threw a improvised encounter at them that was labeled as deadly by a en****er calculator. Not a single scratch on them... aside from the druid getting dragged off and mauled by a bat hound. 

my goal is to spice up the challende a bit, giving them at least a greater feeling of being in danger 

My current idea would be to give caster enemies more spells like counterspell, dispell magic or heat metal. lowering recharge time? Turing some abilities into bonus actions?

What would be your advice?