Game playing

I’m working on a side charity project with a guy I was once involved with. I think we both still have feelings for each other but it’s not relevant to bring it up with each other when we’re communicating about the project.
He is very responsive to me when we chat on a personal level but is very slow to respond on the project i.e. Takes over a week to respond to an email / I have to chase him a lot etc. Because it’s us I tend to take this quite personally.
The project is just about done and he recently emailed me his last part, also saying sorry for the delay and acknowledging his ‘poor behaviour’ on the project.
Because I’ve waited over a week for the reply and all I really have to say is ‘thanks’ I’m holding off on my reply to give him a taste of his own medicine. I’m usually very responsive and can’t really be bothered addressing his apology so wondering if holding off and just saying ‘thanks’ is ok. I realise I am just trying to make him feel how I’ve felt – something I have no control over clearly. Thanks!