[RPG] If a spell states to add “spellcasting ability modifier” do I add just the modifier (wis or int) or the full spellcasting ability (wis + proficiency)

A spiritual weapon (created by the spell of the same name) uses your bonus action for movement and attacking. It does not have its own actions, bonus actions or reactions.

The Spiritual Weapon spell has a casting time of 1 bonus action and range of 60 feet. Breaking down the spell description:

You create a floating, spectral weapon within range ...

So, you use your cleric's bonus action to cast the spell, creating the spiritual weapon within 60 feet of your cleric.

When you cast the spell, you can make a melee spell attack against a creature within 5 feet of the weapon.

If a creature is within 5 feet of where you created the spiritual weapon, as part of the same bonus action you used to cast the spell, you can attack that creature with the spiritual weapon.

As a bonus action on your turn, you can move the weapon up to 20 feet and repeat the attack against a creature within 5 feet of it.

In every round after you cast the spell, for its duration, you can use your bonus action to move the spiritual weapon up to 20 feet and attack a creature within 5 feet of the spiritual weapon.

Since any use of the spiritual weapon spends your bonus action, your cleric still has a regular action (and reaction) available.

This is the wonder of RPGs, they are flexible. But they are not without their trade-offs here.

Probably not an issue

• Spell DCs
• Spell Attack modifiers

Probable issues

• Physical vs Mental
• Saves
• Skills
• Flavour

Let's break these out.

Physical vs Mental

Casting is generally tied to one of the three Mental stats (Int, Wis, Cha). If you allowed an Eldritch Knight Fighter to make their casting stat Str this would probably cause power issues because you would be letting them "double-down" on their best stats. Likewise for Arcane Tricksters, Paladins, Rangers.

Saves

The game technically has six saves, but Dex, Con & Wis are really the most important saves. If you look at the Save Proficiencies by class, each class only gains proficiency in one of these two saves. They also typically gain proficiency in the save that matches their best stat.

So Sorcerers get proficiency in Cha saves making them really good at Cha saves. Rogues are really good a Dex saves, Clerics are really good at Wis saves, etc. If you make a Wis Sorcerer, they become "pretty good" at Cha saves, but also pretty good at Wis saves.

This definitely changes the nature of the class. You normally don't target a Cleric or Druid with a Wis save because it's probably not going to work. But now it's not so clear. This will affect both PCs and NPC casters.

Skills

Available skills tend to be connected to a character's primary stat. So Clerics get more Wis skills and Wizards get more Int skills. If you switch this around, it really reduces the odds of having someone particularly skilled at one of these disciplines. If the Cleric doesn't have a good Medicine score, then who does? Same for Arcana?

Fortunately, some of these can be made up for with specific Backgrounds, but it does hamper specialization usually connected with the given classes.

Flavour

There is a specific history behind the various casting stats. Int casters had to study, Wis casters are divinely inspired, Cha casters are "naturals". When you switch this stuff around, you're also implying a different world vision than the "classic" D&D.

This is not "bad" per se. But it is significant. Why does a low Int Wizard use a spell book? Are they really going to be book worms if they use Cha to cast? If Sorcerers use Int to cast, but don't need books is there a reason for that? If Clerics use Int to cast, then where does that whole "divine inspiration" thing kick in?

These questions can clearly be reconciled, but they are meaningful. They say something about your game world that is different from say a classic Forgotten Realms world.