Battery Range Experience for Specialized E-Bikes

Actually I'm riding in a similar 'race' in November too - ebikes allowed but can't 'win'. The distance is ok - 125kms (78 miles) but there is 2,700m (8,860 ft) of climbing. IME climbing is by far the biggest drain on the batteries. I don't think I could make it on just the internal + RE, but I will borrow a RE from a friend and hope that is enough. Any thoughts on the best way to manage the 2 RE's on such a ride?
I ride an SL4 EQ with an RE. I’ve been less energetic this year than in the past. Maybe being 75 has something to do with it? This means I use Sport and Turbo mode more often. (35/35, 45/80, 100/100)

Anyhow, most of my rides are on a rail trail - long shallow hills - and less than 400 feet of accent - and about 20-30 miles.

I get at least three rides per charge. That means I have about 25-30% charge left when I plug in. So, I get between 60 and 80 miles most of the time. I suspect I could do a century on the right route and weather.

I‘ve never ridden long enough to get below 15% with one exception. I had organized a 40 mile ride for my ebike group. My bike was down to about 25% the day before the ride and thought I’d plugged it in. Nope. I rode to the starting point - about 4 miles - and left the bike powered on while we all gathered. About 5 miles into the ride my batteries got below 5% and gave out. Did the remaining 35 miles without assist. SL!

That was two years ago. I don’t think I could ride that far without assist today.
Any thoughts on the best way to manage the 2 RE's on such a ride?
Were you not in Australia, I would borrow you my 4 Range Extenders :) Now, seriously.

Set your Creo to use both batteries at the same time. Ride all the flats without the assistance if possible. After you did a climb, go OFF to save the batteries even if you are pedalling downhill. Use the next RE either at the 10% of either battery (15% of 150%) or -- if you must -- deplete both down to 5% (8% of 150%) before swapping the Range Extender. The point is: Below the 10% of either of batteries (15% of 150%) the SL is going into the battery saving mode, dramatically reducing the assistance.

Now, why not ride with the "Discharge the Range Extender First" option? It is because the Range Extender would drain just before the most important climb on your route... Guaranteed! :D Use the second Range Extender just to complete your race!

Besides, 30/60% SL assistance (main battery + Range Extender) gives maximum range of 165 km in mostly flat area.