I think your training is along the right lines for those longer distance TT’s. The Sustained Power build with a regular diet of high TSS weeks which as long as you can cope with that sustained load is good training for longer events.
There’s not real absolute need to train at the race intensity you’ll be riding at for the 100’s - the higher intensity rides of the Sustained power build supplemented with longer endurance rides should drive the adaptions you need. However you can create you own sessions if you want - I used the workout creator to make increasingly longer versions of Mount Bond largely for IM training as it is at about the IF I want for that. If you really want to ride indoors at that intensity it might be worth considering.
Having said that the description you give of the outdoor rides are actually fairly similar to the indoor rides of the Sustained Power Build but just longer. If you are finding it difficult to recover from those you may find a better training balance by reducing to intensity to a longer endurance intensity ride. You might find that you recover better and are able to train more/better with that change and build more fitness over the longer term.
With regard to the 12hr I wouldn’t change too much from that sort of training either, rolling consistent high TSS weeks will build the fitness you need, the trick is finding the balance of what is sustainable for you. I’ve only ridden one 12hr TT a couple of years ago and I didn’t train specifically for it but again rode it off the back of IM training which isn’t very different to what you are already doing.