Hi, @matiassaarinen. First, even with more volume/training stress, the easy day/hard day approach still has its place considering that the body adapts during recovery. When you deprive it of sufficient recovery, you can’t expect adaptation to the same height as a body that’s benefiting from proper management of the stress/recovery cycle.
Increasing the stress of your usual high-quality days is a good first step, just make sure not to overdo those workouts. And don’t raise the intensity, add more intervals. The intensity shapes the workout more than anything else, and altering it will and usually does alter the workout’s outcome.
It’s pretty common to see riders advance their training by heaping on extra intervals through a beefed up version of an already demanding workout, but this can require more recovery than usual. This results in a more fatigued rider who then presses onward with yet another tougher-than-usual workout only to perpetuate a cycle that sees him/her dragging after a couple of weeks.
So the challenge is to increase the load gradually and be patient, a bit conservative, rather than bury yourself only to degrade the next week of training as your body struggles to realign itself, or maybe never really does. This is visible when you see otherwise strong riders underperform race after race, ride after ride. So 15% may be manageable, it may not. If you’ve escalated your training load in the past, definitely use that information to guide this upcoming set of increases.
And yes, once your body habituates to one type of stress, you either have to add more of that stress or change the nature of the stress. With Sweet Spot, it’s common to come up against a ‘volume wall’ where you simply can’t do more or longer SST efforts making it necessary to supplement with something else or change your workout composition more radically. Adding a day of VO2max work could be the answer, doing a couple weeks of LSD could work, maybe a week or 2 of sprint-intensity work will catalyze a bit of improvement - lots of options.
And plenty of riders, especially in the 4+ w/kg realm, benefit from training 6 days/week, as long as those days are structured properly and allow sufficient recovery relative to the amount of work being performed. This again makes the case for the hard day/easy day approach, but it doesn’t have to be strictly one-for-one. You can stack a couple harder days together followed by a single rest day; a weekend followed by a Monday rest day is a pretty common way to do this. But often that composition is a hard Saturday and a long Sunday rather than back-to-back days of intensity (though they have their place occasionally too).
My advice is to make your hard days slightly harder as a first step; Tue/Thu and maybe even Saturday too. Then you can tinker with a longer but light Wednesday ride or even the addition of a light Friday ride. Just keep things gradual and listen to your body, cliché as it sounds. It’s easy to dig a hole that sets you back, much tougher to balance the increased stress with increased recovery and keep things trending positively.