1899-1900 M101-1 The Sporting News Supplements would be the best long term investment IMO. This set includes the Honus Wagner rookie.
https://oldcardboard.com/m/m101-1/m1...?cardsetID=961
Also the 1909-13 M101-2 Sporting News Supplements still somewhat affordable and have become very popular of late, led by the early image of Ty Cobb. This set is loaded with HOFers.
https://oldcardboard.com/m/m101-2/m1...?cardsetID=962
Supplements have always been a bit of a nitche, but are moving more mainstream like postcards did 10-15 years ago.