It's probably too strong
Firstly let's compare to what other 1st level domains get.
- Arcana: Arcana proficiency, two Wizard cantrips
- Death: Two martial weapon proficiencies, Reaper*
- Forge: Heavy armor proficiency, smiths tool proficiency, Blessing of the Forge*
- Grave: Circle of Mortality*, Eyes of the Grave*
- Knowledge: Two ability proficiencies
- Life: Heavy armor proficiency, Disciple of Life*
- Light: Light cantrip, Warding Flare*
- Nature: One druid cantrip, one ability proficiency, heavy armor proficiency
- Tempest: martial weapons proficiency, heavy armor proficiency, Wrath of the Storm*
- Trickery: Blessing of the Trickster*
Domains that only get cantrips and proficiencies are definitely worse than your proposed feature, so lets compare with Death, Forge, Grave, Life, Light, Tempest and Trickery. But first...
About spiritual weapon:
Is it better than one attack per round? Well yes and no.
- It uses your spellcasting ability modifier, this could be better or worse but it's certainly different.
- It is a second level spell, that lasts for one minute
- You can't cast an action spell on the turn that you use spiritual weapon
Because it is a second level spell, it is a limited resource that you can't have forever. When a typical cleric decides to cast spiritual weapon, they can't cast an action spell that turn which limits their action economy.
With your proposed feature the extra attack itself doesn't cost any resources. While you do need to use resources to activate it, those resources still do whatever they would have done otherwise - making the extra attack free.
Why is this feature too strong compared to...
As you've explicitly mentioned, what do higher level clerics tend to do with their actions? Cast a lot of spells. So what do they usually do with their bonus action? Not much. They might be using spiritual weapon, however that comes at the cost of spell slots and not being able to cast an action spell. Typically a Cleric won't use their bonus action much of the time.
This ability affects the action economy quite significantly, giving the cleric with this ability a large number more attacks per long rest than most other cleric types.
Compared to Death Domain
Reaper allows the cleric to cast a necromancy cantrip at two targets instead of one, as long as they are next to each other. This might be the closest comparison to your feature however it's definitely not as strong. The effect only triggers in specific scenarios (compared to almost always) and when it is used, the caster must spend their action on a cantrip which is not an overly strong manoeuvre.
Compared to Forge Domain
Blessing of the forge allows you to add +1 to a weapon or armor, which is OK in the early game but quite insignificant in the long run.
Compared to Grave Domain
Grave domain has a significantly better save the dying cantrip, and better healing on restoring creatures from 0. They can also sense the undead. Neither is particularly game-breaking and both are situational.
Compared to Life Domain
Extra healing on spells, nothing too noteworthy here.
Compared to Light Domain
Gets an extra reaction option to give an enemy attack disadvantage, can be useful but reactions are limited resources. You can only use this as many times as your WIS mod.
Compared to Tempest Domain
Gets a reaction damage option. Reactions are limited resources, and you can only use this as many times as your WIS mod.
Compared to Trickery Domain
Can give a creature advantage on DEX saves for an hour, not gamebreaking.
tl;dr - Summary
I think this ability is too strong and versatile into the late game. Most of these other level one features are good or OK in early game and much worse later. The best ones are situational and this ability is almost always good.
If you cast a spell with a spell slot of 1st level or higher and an ally is one of the targets of the spell, you can make one weapon attack as a bonus action. You can use this feature a number of times equal to your Wisdom modifier (a minimum of once). You regain all expended uses when you finish a long rest.