I would suggest advancing yourself in two ways from where you seem to be at with Capuzzi.
The first would be to continue working through a range of studies that are about as hard or a little harder than Capuzzi, that will broaden and strengthen both your bow and left hand techniques (as well as challenge your fingering solutions). Lee 12 Studies (IMC) will develop legato bowing. Sturm 110 Studies,Volume 1 (IMC), Slama 66 Studies in All Keys (IMC) and Bottesini 24 Studies do not ask you to do impossible things with your left hand. Sturm has short varied pieces that also demand dynamics and expression. All these studies are only medium difficulty and you can always slow them down to still get heaps of value. Try and make each one sound different. To me these books are a lot more fun than Simandl 30 Etudes!!
The second suggestion is to find complete bass parts instead of excerpts to study. Excerpts books contain passages chosen because they are somehow difficult. They don't give you the context of the bass line, the style of the piece or speeds. You can borrow or buy CDs of the works so that you can follow the bass part as you listen, then put into practice what you hear. I am thinking of the Oscar Zimmerman books of complete bass parts for Beethoven, Tchaikovsky, Overtures, etc. (don't worry about Richard Strauss until later!!). Try to find parts for Mozart Symphonies 29, 36, 38, and 40. Then Beethoven 1, 3, 4, 5, and 8. Then Tchaikowsky 4, 5, 6 and 1812 Overture. Perhaps also Dvorak Symphonies 8 and 9.
Best Wishes,
DP