Great to hear you have stuck with a consistent program for 3 years. I’m sure you’ve seen some good gains.
As we mention in the book the Transition period can and should be adjusted according to your individual situation. You basic or non specific strength sounds like it is quite good right now so you do not need to start with a general strength program. However you might want to consider who much benefit you will see from increasing non-sport-specific strength much more. For instance: Going from a max deadlift of 50% of body weight to 100% is likely to help you in climbing, hiking, running, skiing. Going from 100% of BW to 150% is going to have a smaller transfer to your sport and going from 150% to 200% of BW is probably not going to transfer into improved sport performance and may even be detrimental to your performance. This is very individual and you will need to decide where you are on this spectrum. Just be advised and don’t get caught into the cycle we se so often where people (in gyms usually) get hooked on improving some non-specific strength as an end in itself.
You will probably benefit from shifting to more sport specific strength training earlier in the training cycle and doing more of a max strength maintenance program 1x/7-10 days.
As for the aerobic end of things: By now you have a good idea of the aerobic training load you can handle at the top end and one that you can comfortably manage to start. Our 50% rule that you are invoking here is not cast in stone. It is a relatively safe guideline that works for most folks and won’t overload them. Use you training logs from the past to see how you ramped up before and how you managed with that ramp rate.
As you become fitter each one of the training cycles it will become increasingly difficult to move the fitness bar higher. It will take more time and more intensity training to increase aerobic endurance once you have a good base established. You may need to up the volume 10-15% this over last year but again, look back at your logs for feedback you were giving yourself as to how you handled last year.