I have a 2019 Roubaix, so 2 generations behind the SL8. It has 11 speed Ultegra Di2 and cost less than that 105 mechanical SL8(!), so if I were to put myself in your shoes, it would be a tough decision whether going electronic is worth it, especially with big discounts on Di2 kits at places like Merlin. That doesn't stop me from imagining upgrading to a SL8, as I would absolutely love having fender mounts and gravel-potential. Hoping they'll sell a non-S-Works frameset or have a sale next year and I would seriously consider swapping my UDi2 over.
Another thought about what to do with that money: the biggest upgrade I made to my Roubaix were deep section Light Bicycle wheels (50mm/56mm f/r depth and not far off 1300g) over the cheapo wheels the lower-tier Specialized bikes come with, which I think is a better use of ~1k USD than springing for electronic.