Please note my actual suggestion was just to grant advantage on the next attack, not include damage bumps at higher levels.
In essence, this is sort of combining True Strike with a weapon attack, but the advantage is for the next attack. Also, I didn't say this, but you should include "the next attack made against the target before the end of your next turn has advantage." But maybe the 1 round duration makes that unnecessary?
Oh, you were saying you wanted it like the GFB and BB cantrips, so I assumed the damage boost at 5th, 11th and 17th like those cantrips was included. I definetly don't think taking those out leads to a better solution.
No, it still isn't ideal, but at least now it requires your action each round and is considered Casting a Spell instead of taking the Attack action, so War Priest's can't use their bonus action in the same turn for another attack.
Why would that be a problem? War Priests aren't exactly breaking the game with their signature ability. And we have a lot of subclasses that now allow "Cantrip + attacks" already.
The problem is more GFB and BB don't have inherent boosts from features like Divine Strikes and their damage depends on certain riders. Blessed Weapon has no riders for the extra damage. Maybe if the next attack (which has advantage) hits, THEN that attack does extra damage?
You could do that, but I don't think I'd want to make it that the extra damage comes when someone else hits. Which gets into....
So, what do you want this cantrip to do? So far you have:
1. Shed light
2. Make your attack magical
3. Increase weapon damage
4. Provide advantage on an attack
All of which is a bit much for a cantrip IMO. Most cantrips only do 1 thing, sometimes 2, but 3 is rare IIRC.
I found the cantrip. It already shed light and made the weapon magical, I felt that wasn't enough and people seem to have agreed. It is also weird you call out your own suggestion as a problem because it adds too much.
But, what is the goal of the cantrip?
To give a melee cantrip for Clerics with Divine Strikes.
I find it notable that there are 9 out of the 14 clerics that are theoritically supposed to wade into melee (Death, Forge, Life, Nature, Order, Tempest, Trickery, Twilight, War) because they get Divine Strikes to improve their melee options. And of those 9 only 4 of them get martial weapons. Meaning that there are 5 subclasses of cleric, meant to be in melee, who fundamentally don't have good tools. A trickery cleric for example has Divine Strikes to give +1d8 at level 8, but their best melee option is their tertiary strength stat and a 1d6 weapon if they use a shield (which they should). This gives their melee option, which they are highlighted as having, a likely best case scenario of 1d6+1d8+3 or 11 damage (And that is assuming a 16 strength, a high wisdom, and a good con). Compare this, not even to Toll of the Dead, but Sacred Flame which is not highly regard, and if they switch to Blessed Strikes, then they could do 3d8 or 13.5. Instantly better, and ranged. And even if you extend this out to 14th level, you are likely not raising your strength, so you get 1d6+2d8+3 or 15.5 vs swapping and getting 5d8 or 22.5
If they were a light cleric? Sacred Flame can do 2d8+4 or 13 damage. Without having to swap, and will only improve if they have a higher wisdom. Best case at the end of the line is 4d8+5 for 23.
Now, take one of those 4 classes with martial weapons. Let's say you go with a War Cleric, who has multiple abilities to use weapons. You grab a Greatsword, forgo your shield, and at 8th level you can deal 2d6+1d8+3 or 14.5. They are only dealing 3.5 more damage than the sacred flame swap, only 1.5 more damage than the light cleric, and they have dropped their shield and are in melee. And at the highest tier? 2d6+2d8+3 or 19 damage. Less than using the cantrips.
And this is comparing with the d8's of sacred flame or a weak Toll of the dead. The strong Toll of the Dead, which is incredibly easy to get because clerics often do not go first in combat, does between 17.5 (2d12+1d8) for the swap or 17 (2d12+4) for the light cleric, or any other caster cleric. And if you go all the way to the end line? 31 damage (4d12+5)
But the majority of clerics are weapon users. The majority of them are encouraged by their base feature to go into melee and are told it will be effective. That is why when I saw this cantrip, I was immediately hooked with the idea that I wanted it in my games. I wanted a way to make the War Cleric or the Order cleric fulfill that goal of being viable in melee, instead of them being better off in simply swapping for Blessed strikes, getting the +1d8 to cantrips and all spells, and just ignoring any martial combat .
TL;DR I want a reason for Martial Clerics to attack in melee instead of taking Sacred Flame and Blessed Strikes.