I’ve got a few thoughts here.
One is that while training in the TT position is the most specific and finding your “TT position FTP” can be helpful in figuring out exactly what your limits are there vs. in a more comfortable upright position, triathlon (especially for the half and full distance) isn’t really about going as hard as possible on the bike. Your race power is going to be somewhere in the endurance to tempo range, so I think you’ll find more advantage from raising your FTP as much as possible and also, separately, getting used to the TT position than you would from training entirely in aero and trying to raise your aero-tested FTP.
Along those same lines, as @chad, @Jonathan, and @Nate_Pearson experienced in training for a 40km TT, knowing that your position is going to be uncomfortable when you train is a real drag on motivation / discipline to train consistently for months. They talk about this in the podcast on planning your year, from around 46:30-48:00, and suggest that if training in the TT position feels like a drag, you’d be better suited to training comfortably and doing a little bit of focused work on adjusting to your TT bike:
As for what plan to do… I think my vote would be to finish SSB II and then go on to do the half distance base / build / specialty. You’ve done six weeks of dedicated sweet spot work, and SSB II actually starts to mix in some variety of effort and give you a break from just doing one kind of workout all the time. If you went straight from SSB I to half base, you’d do a few more weeks of dedicated sweet spot work before you started to see any variety. I think variety now would be better, and then more focused sweet spot later.
Now, that covers the bike training. It sounds like you are already doing triathlons, so you know where you stand with running and swimming. If you don’t, then spend the time during SSB II to get used to running regularly and figure out your swimming. If you do, then don’t stop running and swimming during SSB II, but focus on easy miles (or train for something like a winter half marathon, just keep in mind bicycle TSS and running TSS are cumulative).