To be fair, the best improvements you can work on a JF 5-digit are actually internal:
1. Date wheel (go gen if possible, or ask BK for one of his overlays, as they are very well regarded).
2. Reverse wheels (unless you want that red paint on the rep ones flaking all over your SH3135).
3. Date change mechanism (some people say the latest SH3135 are way better finished and don't have any problem with the date change, though).
Then you can consider the crystal and maybe the insert. The dial and hands are just superb so I don't think going gen on that would be necessary.
In terms of the SEL, I would try different springbars first before doing any drastic changes. Mine had average SEL fitment, until I realised my gen Black Bay had an extra set of springbars (they were in the NATO strap that comes with it), so I replaced them and look at how much nicer is the SEL now:
I'm sure you can get a pair of gen-spec Submariner springbars and this
should give you a quick and cheap improvement.