I was taught, many years ago, that "I would like to..." means you want to in the future. If you are writing or speaking and thanking someone (on a plaque or otherwise) in the present, "I thank you for your support...".
Therefore, "we thank you" is appropriate for the plaque. Although I agree with the comment "In recognition of your...".
I cringe when someone stands on a stage and says, "I would like to ..." because they're doing right now. I never heard the "extreme politeness" explanation, but "tentative," yes, because you want to but aren't going to thank the person right now.