I used to do ad scheduling 10+ years ago and at least in those times the ads has NOSCRIPT tags.
But for it to really work you would need to store a cookie to correctly redirect the user.
I like your idea of rendering the ads on the server side, but I would hope they would have super low responsive times. Or at least low timeout on your side.
You can render client side too, the key is no external JavaScript is being trusted to run on the page.
That ofc means the common practice of advertisers pasting a JavaScript snippet, the network doing a review process, then rendering that snippet as an advertisement on some property would not be allowed on this Ad Network.
But for it to really work you would need to store a cookie to correctly redirect the user.
I like your idea of rendering the ads on the server side, but I would hope they would have super low responsive times. Or at least low timeout on your side.