Man, what about switching to a different style of slide altogether? Those full extension ones always need so much room. Have you looked at any of the low profile side mount options? Sometimes they'll work with thinner material because the whole mechanism sits differently. Or is the client dead set on that specific undermount look?

Ugh, it's like that with everything from phone plans to gym memberships now!

Wait, it was from 2021 and you still tried it? You're braver than me.