Will you be keeping the Sienna for at least a few more years? If so, this maintenance item makes sense. If more incentive is needed, it might improve your gas mileage.
As for brand and type of spark plug, stay with exactly what is in there now. You do not need to be ‘upsold’ to a fancier plug. If more incentive is needed, the same plugs should give you another 125K with essentially zero risk of an unexpected compatibility issue (and the plugs are likely cheaper than the fancier plugs).
And regarding cost and DIY, you probably should not try this yourself unless you are reasonably comfortable with this type of work AND have some experience with what to do when things don’t go as expected. For example, are you ok with the idea that a plug in ‘the back’ might be really stuck, or breaks while you are trying to remove it?