How did you balance it? I've found that if a creature can survive a party nova and dish back, PCs are going to die. If it can't, then I need multiple encounters for that "day."
With 6 players, I needed a bit more. 1st monsters are rarely alone. I often put low level threats.
2) I do not allow the get in, get out that most DM do. Once you are in a lair, if you flee, you can be sure that search parties will be coming to you. So players have learned to play accordingly. This forces them to use such combat options as the dodge action to mitigate hits a bit more.
3) I have modified some monsters. Not much, but some. The veteran for example. This foe uses a long sword, two hand or one hand but does not carry a shield? Add a shield and now the veteran lost 1hp of damage on average but gained 2 pts of AC. That 10% alone makes players cringe when they see one. Or give the veteran a two handed sword and raise the potential damage.
4) I have modified solo monsters and I put fights with minions before any solo fights.
My mod for solo
Single encounter big bad guys
Add 1 feat/ASI per 4 CR (save the first four)
Add 1 legendary action per PC above 4.
Multiply HP by 1 +0.25 per PC above 4.
Add 1.5 AC (round up) per PC above 4.
Here is a modified lich my players defeated at 17th level in a previous campaing.
AC: 17 (natural armor + dex) Now 23 (+1.5 AC x 2 for players number, Staff of power and +1 ring. It could go up to 25 for one attack because of the shield guardian and I am not counting shield. Since shield was an at will power, it became and AC 28 and 30 for one attack)
HP: 135 (18d8 + 54) Now 238 for a 1.5 multiplyer for number of players and tough feat (+36hp).
Spd: 30' (unchanged)
Stats: Unchanged (decided to use feats instead...)
Added: Warcaster, Elemental Adept Fire (that one was a surprise for my players), Tough.
Mage feature added: Shield and Mirror image at will (I counted that one as a feat)
Spell list was changed to reflect the added mage feature and feats.
Cantrip: Removed prestigiditation and added Fire Bolt. Rose number of cantrip to four and added Green Flame blade
Level 1: Removed shield and added disguise self (Lich used it to appear human as she did in her life.)
Level 2: Removed Mirror image and added Misty step instead.
Legendary action Rose to 5 (again 6 players)
Lich was also using Staff Of Power And a Shield Guardian.
Note that damage potential is unchanged. Only defensive options.
Add in the Mythical ability that came with Theros and your BBEG or Solo will be much more memorable.
I used a sword wraith captain to which I gave this treatment and the Theros mythic ability and the fight lasted 13 rounds with 8th level characters. They won but it was truly an epic fight.
5) Creature usually use their treasure if they can.