Dear Mick,
First of all ((((((( hugs))))))).
Nothing harder than to find a Song that tells of the depth of Love and gratitude you feel for someone so special who is preparing to travel on ahead.
There are already some lovely suggestions.
I can think of many songs but......
I am sure that when the time comes Mick you will hear your Heart and it will tell you what Song needs to be sung and it will be the one that best describes this lovely Woman and how you feel about her.

Your description of Louise is beautiful. May the time she has with you all be Peaceful and Tender.
Light, Love and Blessings to Louise, Yourself and your Wife and all your Family Mick.
Holding ye in my thoughts.
Jude