Better warmup

Attempting my 1st full Ironman CDA on 6/29. I have completed all shorter distances multiple times within the past few years with the exception of the full which I will hopefully do this year.

I was planning to do the Little Smokies 1/2 on 5/18 as my only warm-up this season prior to the full, but read about the Great Lakes 100 mile tri (1 mile swim, 84 bike and 15 run) on Slowtwitch and am now thinking that would be the better warm up, more so mentally, than physically.

My concern is the 100 mile tri is only 3 weeks prior to CDA and am thinking that this would not be enough time to recover and get any additional valuable training in prior to Ironman CDA, meaning the 100 mile tri would be my last significant training session and I would recover right into the taper for the full…

Thoughts? Stick to the initial plan or go for the 100 mile tri?

Thanks for any advice!