Good idea. But why make it soooo long?
Most monsters are fine. Only solo ones are a bit lacking if they are the only encounter of the day.
Again here are the things I am doing and it works out quite fine. Try it.
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.