Start with baroque stuff and work forward historically. In a typical Handel score there are only 4 or 5 staves altogether, with at most one in an awkward clef (viola in the alto clef) and one transposing instrument (usually horn or trumpet). Mozart and Haydn seldom had more than about 10 staves in early works, maybe 15 in later ones. Don't bust your head if you have trouble sight-reading R. Strauss works with 25 staves and 10 transposing instruments - only one or two people per generation, worldwide, can really do that.
The most difficult tranposing instrument (as you, dear Richard, know all too well!) is the voice; how often does the singer say to the long-suffering pianist "this is not in "my" key", thereby tacitly expecting him/her to transpose ā l'instant?...Best,Alistair
WAY too often