It's not surprising you perceive your phono sound as "thin." The SME 3012-S2 is light for the Denon 103Pro, making the two a marginal match. The SME's effective mass is 9.5g. The Denon's actual compliance (converted to industry standard test @10Hz) is just over 10. You're just on the high side of the cartridge's sweet spot with that tonearm. You're also tracking it a little heavy. The Pro version of the 103 is rated for 2g +/- 0.2g, but you probably have to in that combination.
The SME comes from the era of high compliance cartridges. If you want a Denon 103 sound character in that tonearm, the scarce and more compliant 103D and even scarcer 103M would be a viable match. The current production DL304 might work, on the low side of that cartridge's sweet spot in that arm. If you peruse the cartridge database at vinylengine.com, just remember that when using the resonance calculator, you have to multiply Denon's compliance rating by 1.8X to convert Denon's 100Hz test figure to the standard 10Hz reference for compliance.
If you want to use a Denon 103 or 103Pro with your outstanding *first turntable!* a tonearm with an effective mass between 15 - 20g will work beautifully. An affordable and wonderfully compatible tonearm in that case is the Thomas Schick "G" length.