If you are basing it that way, then your ON season is more about
specificity to the race/distance that you are training for, ie marathon specific training......or put it this way you are training to peak at the correct time. And you can still maintain mileage to be fairly high during OFF season (eg your 6 months FM training block is aim to peak at 100km/wk, while your off season is no less than 50km/wk i feel that say ~ 50% is the bare bare minimum actually for seniors.....preferably no less than 70km/wk if wanna regress vol during "OFF" season........
Then seriously it is just a "phrasing" thingy and we are more or less on the same page.
For the term cumulative fatigue - i dont know leh.....so far i feel that deload week every now and then (i dont even follow a very specfic schedule, like 3 ON and 1 deload) really takes care of things. For me i base off feel, as i do not follow any training plans . However i do follow what i term as a semi structured one, as in i know i am following a structure more or less but it is based a lot on feel, i like this method coz long story cut short, my preference is this rather than a specifically set pace or a set number of sets. But dont get me wrong, it is not going to be radically different anyhow hantum one.
This is also my preference based on my strength/calisthenics routine - i feel based on feel and just semi-structuring it to be not totally rigid just works better for me.
Also concerning (cumulative) fatigue i feel is based a lot on recovery rate - ie all of us are different. Pls dont over read it and i dont mean anything in particular, but i believe your sleep hours per week is less than mine so that might be one significant factor. Nutrition aspect also. These are the big 2......there are of coz others.