Your plan looks pretty good.

The intervals I would concentrate on would be steady state/TT efforts at your LT heart rate or slightly below/above (or somewhere in the neighborhood of 1-2 mph faster than you typically can ride for long durations) -- I think these will provide the most benefit for your event and your goal of a higher avg speed over a fairly flat course. You might start with 2 x 10 minutes with 10-minute recovery between and work your way up to 2 x 15, 2 x 20, 3 x 10, 3 x 15, and so on, and you can also vary the recovery time.

Since the distance won't be a problem for you, you might consider reducing the number of really long rides, with some shorter rides (still long though) that incorporate intervals within the ride -- for example, do a 4 hour ride, and 2 hours into it do some steady state intervals of say 3 X 10 minutes with a 10 minute recovery between them (and the intervals would be at 1-2 mph faster than your "race pace/long ride pace"). Then the remainder of the ride would be back at your normal race pace. You can vary the length of intervals, the number of intervals, the number of sets of intervals (1 set an hour into the ride and 1 set 3 hours into the ride, etc.) so your rides are always a bit different and continue to be challenging.

Also, you might substitute some of the long rides for faster group rides.

Don't overdo it too close to the event -- the 300km event may not be necessary -- make sure you have an adequate taper.

I would do your core strength exercises at least once a week on an on-going basis (but discontinue any "heavy" weight training as per your schedule). Add some neck exercises since you will be using aerobars and get comfortable riding in the aero position well in advance of the event.

Finally, make sure you get plenty of recovery days in between your hard workouts -- take some really easy short recovery rides and take days off. You'll want to include some recovery weeks as well -- every 4th week is typical (where you just do recovery rides or take days off or do some easy cross-training).

Good luck and keep us posted on your progress!