As noted, Power Word spells where still fast in the old game. So how about we mess with that, instead of 4 rounds.
So
(a) Start with gritty rests. It takes a week of vacation to get the benefits of a long rest. During this week you can engage in downtime activities, and you gain the benefits of a short rest each night. If you do anything as intense as fighting, each time you take damage, and for each non non-cantrip spell on a day, that day does not count, and you lose 1 additional day towards your long rest (capping out at 7 days).
For slow spells that take more than 1 action? 10x casting time.
For spells that are 1 bonus action or action, start with the number of actions (0 for bonus action spells), then add 1 action for every component (VSM) the spell requires.
That is how long the spell takes to cast. You can only spend 1 action per round on casting a spell (no action surge speedup).
Removing components with metamagic does not speed the spell up, but quicken spell removes 1 action.
Each turn prior to the one where you cast the spell, you make a spellcasting attribute check (you have proficiency) or against a DC of 10+twice spell slot level.
Fail by 5 or more, or natural 1: Spell fizzles and fails.
Fail by less than 5: You don't progress on the spell; the spell requires another action.
Succeed by less than 5: You continue.
Succeed by 5 or more, or natural 20: When the spell is cast, it has +1 to hit, +1 DC and is cast as if it was using a slot 1 level higher. If it is a cantrip, it is cast as if you where 1 tier higher in level (progress beyond T4 is usually obvious).
While casting a spell, if you take damage you have to make a concentration saving throw; on failure, you the spell is lost as well as any effects you are concentrating on.
If the spell is lost due to stopping casting, concentration check failure, or failing a spellcasting check by 5 or more, you lose any slots and material components for the spell.
---
So magic missile has 2 components, so it takes 3 actions (and hence 3 rounds) to cast. If cast using a 1st level slot, the DC is 12. A level 1 wizard with 13 int has a +5 bonus. Each round they roll 1d20+5.
So at +5 bonus against DC 12 we get:
On a 1, the spell fizzles.
On a 2-6, they fail, and the spell requires another round.
On a 7-11, they succeed.
On a 12+, they succeed, and get a bonus.
After 2 rounds...
6.25% 0 progress has been made; delayed 2 rounds.
9.75% spell has failed.
6.25% spell casts normally next round.
22.5% spell casts with a +1 bump next round.
20.25% spell casts with a +2 bump next round.
12.5% spell is delayed 1 round
22.5% spell is delayed 1 round, but also has a +1 bump
---
This makes combat magic less practical, but it at least makes up for a it a bit. It also provides a minigame (roll to see if you boost your spell, or it fizzles, each round), so the spellcaster isn't doing nothing at all on their turn.