Trying to avoid being dinged for PBN-esque practices

My client has 5 domains, 3 of which (we'll refer to as C, D, and E ) we are being paid to optimize.

The first 2 domains (we'll refer to them as A & B), are the oldest and are no longer consistently updated but still get leads daily.

The client wants to link A/B to C to help drive the leads. C is the same industry as A/B, just a different business model (think consulting vs an ebook store).

I can't emphasize enough that this is strictly white-hat ONLY.

Is it safe to have multiple links on relevant pages from A/B leading to C? What do I need to watch out for? They're all hosted on a VPS and have original content separate from each other.

Tips very much appreciated, thanks!

Why not? It's a common practice, even among some of the biggest media companies in the industry. It's especially effective if it contextually relevant to what you are pointing the link too. If A/B sites have good authority, may as well drop a link in each footer to site C. I would try to stay one way. Don't interlink all properties. For example. Link from A/B > C, but don't link C to A or B.