This assertion isn't even on the same planet as this thread.
I think that ECMO3 might be reading several different versions of the suggested class and claiming that the intent is that the class will have all of those abilities, rather than just the abilities of one of those versions.
You say you want to "blend spellwork with combat" if that means casting a full leveled action spell on the same turn you attack, that is 2 actions in one turn, and doing it repeatedly while having access to heavy armor and martial weapons it is WAY, WAY overpowered compared to other 5E classes. That is essentially 2 actions on a turn and will far, far eclipses the damage a Paladin can do with smite.
What level would the suggested class be able to be combining the weapon attack and spell, and how many times would they be able to do it?
Does the suggested class have multiple attacks with additional damage riders like a Paladin, and thus have effective damage on those rounds when it isn't burning spell slots? Does the class get access to heavy armour as part of the base class like a Paladin, or does it have to expend resources to get that?
Once you have understood all of those, do you still thin that the suggested subclass would be overpowered, and if so, why?
(The class as a whole; not a single ability taken in isolation with no context.)
There is math involved here. A 3rd-level spell as part of a melee attack action should be worth 4d8 in 5e. In earlier versions of d&d, without bounded accuracy, doing more than this would not have been as big a deal but in 5E it is.
What level spell do you think would be needed for a fireball that does an extra d8+5 to one of its targets?
Now if by blend you just want to be able to do both in the same fight - then just attack one turn and then cast a spell on your next turn. Wouldn't this meet the theme - Attack and then 6 seconds later cast a spell? If this still does not fit the theme, another alternative is to use ready action on your turn to attack right before your next turn. So your reaction attack would come out and then your action would follow immediately. Thematically you could even say your attack is releasing the spell without changing the actual game mechanics and if you miss you could use your action to ready again instead of casting. You can do these things already, they fit the alleged "theme" people claim they are looking for, and they are balanced with the other classes in the game.
There are already five or six classes/subclasses that work by casting spells and making attacks separately, and at least two that can do that in the same round.
Certainly when I designed my suggestion, I was working on the basis of something a little different.