Do your casters have 16 in their casting stat (Cha for Warlock, etc)?
Your spell attack bonus (the thing you add to a d20 roll to hit) is calculated as your proficiency bonus (+2 for 1-4th level characters) plus your casting stat modifier. Weapon attack bonuses are calculated the same way, just using Str or Dex instead of a mental stat. Thus, equivalently optimized characters (with 16 in their main stat) should have the same bonus to hit. Additionally, you can make these more likely through high ground, bless, etc, as you've mentioned.
Your Spell Save DC is calculated as 8 + proficiency + casting stat modifier (should be 13 for 1st-3rd level characters). These spells will force the enemy to make a roll of the specified type (dex, con, or wis typically) and their chance of success depends on their stats. Thus, against an enemy with high dexterity (e.g., nimble goblins) you wouldn't want to cast a spell at them that forces a Dex ST. But, since Goblins have 8 Wisdom, a spell that forces a Wis ST would be *more effective* than an attack roll spell. Choosing the correct spell for each enemy is important.
I wouldn't compare *your* chances to hit with the *enemies'* chances to hit. While they should be similar, that's comparing apples to oranges. You want to compare *your* chances to hit with your *party members'* chances to hit.
