Louis XIII's Mysterious Letter (1635)

There is a letter from Louis XIII, dated 6 April 1635, which is written in the normal alphabet but is full of nonsensical words except for a few French words. (I learned of this from David Chelli and Cipherbrain.) I added this in a section in "French Ciphers during the Reign of Louis XIII".
Here is my provisional transcription:

Mon Cousin, bac racle suroaert brancher
guignanocher rip goelle harpe volduc
hagardement son bac bobe her rabo
en vol baratre her Spir, siron pincer
her lily ser, cest basco rideray son sol
flaih poignart son vol tigre Capitulo
fauche midot siron ficher a vol Stellee
bac alarme a son broche ramo sol trepont
los ridant son taf sol branchoit lily ser
ton vol inquiet her menin citro Capitulo
et son sol abonast holta hillot, cest poursevoir
son pinson duc Chatou, nono los racke ride
a los semsom margot sol me pellera, cest

nen ay tor profite pincer a Astolfe her
gorion her vol soter, circo margot vol
Capitulo pousera verie, cest vol poudreray
fon sol ne bulte a son ferre, ny ne vol
mare son vol moins son sol armera
rip go son nono joute son go Stercol vol
ne me roguette for, cest ne rabo vol
brancheray tor pinson inerne venant
son tost her basco marert midot
tarespant joue cest rodilloraq vol octro
exhalt tour basco brosse en sa croche
Louis
a St germain en laye
ce 6me Auril 1635

Some ideas are proposed in comments to Cipherbrain, but no conclusion is reached.
By the way, Klaus Schmeh announced Cipherbrain will be closed at the end of the year. Cipherbrain manages a very lively community where people from all over the world gather and discuss various ciphers or other related mysteries. I hope some other platform will be found to host his blog.