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I live in Iowa, USA - I wish I could send the Excel sheet to get a feel if this should be in an auction.
IMO your best bet is to take selected volumes (some of the more interesting volumes or the ones where you/he think more valuable items are contained), and bring them to a large stamp show and get multiple dealer opinions. Take cellphone pictures of as much as you can to also have if needed.
You'll quickly get an idea as to whether there is any interest or you get brushed off. I would recommend having your brother accompany you so he experiences this firsthand.
There are several large stamp shows upcoming here in the midwest within driving distance. Each of these shows will have 35+ stamp dealers:
INDYPEX - Indianapolis, IN. This coming weekend Oct. 6-8.
CHICAGOPEX - Itasca, IL (likely the closest to you). Nov. 17-19.
St. Louis Stamp Expo. March 22-24, 2024.
Especially for your benefit, if there is disagreement on value assessment between you and your brother, I think he needs to hear it with his own ears, not filtered through a third party.
Also, even if he is right with respect to catalog value or market value, there is a HUGE disparity between that and what your average collector will pay, let alone what a dealer will pay on a wholesale basis.
If I were closer (southern Illinois), I would offer to come by and give some recommendations. I think a larger regional show like the ones above would be a good starting point.
I will be at CHICAGOPEX from Thursday through Sunday should you decide to attend and you would like some advice before you interact with the dealers.