Hi there @Daniel Jepsen,
@retention gives a lot of great insight and help here.
Knowing a little bit more of your solution, I would say although you got consent from those 4,100 and are okay to email them, there could be a possibility of poor deliverability since it was a year ago and your customers could have forgotten they told you this.
For those new 1,500 customers who you haven’t gotten explicit opt-in affirmation from, I would take the Klaviyo form or transactional strategy that @retention gave.
Even though past customers have given you consent to receive service reports this is not the same as explicit marketing consent and would take @retention’s advice where you lack explicit consent!
Hope this helps!
-Taylor