Are you sure using DnD stats without changing anything else would work ?
We're fighting a bulette at level 4 and 4 harpies at level 3. Too powerfull and/or too weak ennemies if I trust the CR. In other word, unappropriate ennemies...
What level should be a party of 1, a party of 2, a party of 3 (and so on) to fight 2 ettercaps and 2 phase spiders with DnD stats ?
You obviously know DnD better than I do but if I'm trying this specific combat in an encounter builder for DD5, it does not seem easy to balance... - party of 4 (average if everyone is level 8) - party of 5 (average if everyone is level 7) - party of 6 (average if you're level 5) - party of 2 (average if you're lvl 14...)
Positive. I tested it several times with different characters.
Let's just take the spider matriarch battle. Characters were level 4 by this point. Rogue, wizard, barbarian, druid, fighter, and ranger. They faced 2 phase spiders with typical stats for phase spiders plus their ability to do ethereal jaunt. I gave the matriarch double stat values, so double HP, double attack value, and double the attacks per round, plus 1D extra for damage. I gave the babies a quarter of the standard value because they just hatched.
Most of the party was taken down by the end. One person was left standing when the last spider fell. I didn't have the spiders porting all over. They would disappear, and then phase back and pounce. The matriarch focused first on hatching the eggs. Then they swarmed the party.
Take the first battle at the gate. Fighter, cleric, ranger, Rogue, wizard and another fighter. What makes the fight right now in the game so easy is that they don't attack you. When we did this fight, they fought back. I used a goblin chief, and a bugbear chief. Then I used normal stats for the other goblins and the worg. The only thing I added was I made one of the goblins a spellcaster with basic spells. That was to mirror what they had in the game. It was still a somewhat easy fight, but it was actually more challenging then what you have in the game currently.
The skeleton scribes took down 2 level 2 characters out of six because they were hurling spells like magic missile. Over and over again I've tested these encounters with proper rules and stats. They are insanely difficult for four characters but doable for 6.