Minimum training maximum outcome
The first thing to say, is that your goal of a 6hr century "without too much trouble" is quite ambitious.  Many cyclists never manage a 6hr century, and even for some quite fit riders, they would really need empty themselves to do it solo.

Training protocols are something of a grey area at the moment, with most coaches still prescribing regimes based on some variation of the books from Friel, Coggan or Carmichael.  

However new research over the last few years suggests there may be more efficient approaches.  As you have already found, HIIT shows good results for minimum time investment, although it is still far from clear which combination of work/rest yields the best results.

Polarised training regimes which are based on the intensity distribution of elite athletes have also shown good results, but there is limited information available on how this applies to athletes with less training and recovery time available.

Without a coach to decide for them, the best thing a self coached athlete can do at the moment is read/learn as much as possible, and then decide which approach they feel best fits their goals and restrictions.  Sadly there are as yet a lack of good books available that thoroughly discuss reverse periodisation, HIIT and polarised training.

To me, it sounds like you are already making good use of your available training time, but without making more time available for riding, I think you may struggle to reach your goal.