Unless you're trying to really boost more bottom end from 6.7 and 6.2m, then the ideal size ST (especially considering the Bay chop vs windspeed ) for your weight/sail size combo is the ST104. If you really wanted to adjust your quiver a little (upsizing your middle board, see below..) and run the ST in even lighter conditions, then the ST115 could also be on the radar. All things considered, looking at the ST as a stand alone choice for 6.7+6.2m /80kg/Bay chop, ST104 still has my money.
Considering you're hooked on the E74 and that's not changing, then typically the "85lt" (2007 = K87..) Kombat would be the logical intermediate "size" choice between the E74 and ST104, especially when considering ride style/rider weight/local conditions. Being a faster and more efficient (as crossovers go..) design, the newer K87 (c/w K85) will offer an easier access to freestyle type performance, while still being small enough to be a realistic wave ride for 80kg riders.
You might want to consider a slight diversion from typical, and consider the larger K97 if you want an increase in Freestyle option.
By size, 97 looks more out of sync with the E74 and ST104, but when measured by useful range and ride/performance, it's not as illogical as the numbers alone indicate...
The current K97 is definitely one of the "magic" boards in the 2007 range that delivers a lot better than it should, and is unquestionably an ideal bay board for anything short of full-on Bay conditions.
Definitely worth a demo, and if you know (or know how to track down) Peter Hands(~85Kg), then he'll certainly give you the low down on his local K97 Bay experiences; if you promise not to destroy it, he might even stretch to letting you ride it
Cheers ~ Ian