My plan to control Ethel was not very successful. I got only a 40% chance to apply control spells (Hold Monster, Polymorph, Fear), even with disadvantage for Ethel, and was not willing to waste time and lvl 4/5 spell slots for it. I have + 14 to spell attacks, how do you get more?
For the Control Spells, you need Spell Save DC not Spell Attack. In your Spellbook it's the little Shield symbol on the top right, if you hover over it, lists what adds to it, like for Spell Attack it's your Spell Modifier, Proficiency, but also things from items or consumables like Elixir of Battlemage's Power - some items give both attack and DC, others give only one. With Spell Attack you role against the enemy's armour class, with Spell Save DC they roll against your value with their bonus derived from the ability shown on the spell. Most control spells have a Wisdom Save, since many caster type have high WIS, they are more likely to resist your control spells.
But the big brutish melee types usually don't have great WIS and if you put them into my favourite Hold Person, it guarantees your own melees crits against them, which is nice especially if said opponent has a high AC.
I find damage spells to be sometimes less reliable, e.g. a glyph spells failed to trigger, because it was placed on a slope and the enemy was standing within the glyph, but slightly higher.
