You're right, I didn't describe my activity level at all - I'd say that whatever the duration I will be working fairly hard for the whole time, with the level changing based on what's maintainable for that duration. Not sure that helps very much though!
I should also add that I do exercise most or at worst every other day (1h or so of reasonably hard riding at lunch time), though with the recent rain I've been doing less and so am suffering from the same issue as a the OP - I should either up my basal slightly or sit on the turbo trainer! Or pray to the sun Gods perhaps
I also completely agree about stacked days having much more effect. While I said I might knock 1U (~10%) off for a single 4h ride, if I do this 3 days on the trot (thinking about commuting I did a few weeks back, all going reasonably fast but I knew I had to do it for 3 days so not as fast as I possibly could) I definitely needed to reduce basal by the end of the week (down to 7U, ~20%, reduction in the morning and 8U, ~10%, reduction in the evening.)
My 1U/10% reduction in morning basal for a 4h ride also is probably too small a reduction in reality (I imagine it should actually be fairly linear with total exercise), but I'd prefer to run slightly lower during the day if I'm stopping (e.g. going to work) as I can always eat a bit more, rather than risking running high and needing to bolus more, which can cause issues with too much IoB (as well as hunger!) on the way home. As the rides get longer I'm less keen on needing to top up like this so I do actually make a more significant reduction to basal (and probably tend to eat less on those days than on a normal day in the office, simply because carrying food and eating it isn't that pleasant.) I do make up for it in the evening mind you when I can again take some bolus.
For a long day out (say 10h in Z2/Z3) I am reducing morning basal by 50%-65%, which works out at a similar number to your 25% reduction. I could probably reduce evening bolus more (and would do if didn't have a ready supply of food), however, this then reduces basal IoB the next day, so it's easier to simply eat more in the evening after a long ride (and I want to do this too as I'm in a calorie deficit) and just make a slight reduction that evening to taper back into normal life the next day.