I think the idea of the separate pattern is a good one. You could also name it appropriately so you know what’s going on when you get to it.
I would trigger the riser with a “1st” condition, so if you accidentally forget to queue a new pattern the riser won’t repeat (unless you’d prefer it did?).
I would then personally use Per Track scale and set the Ch Len to however many steps you want the riser to end at. You wouldn’t need to actually set any of the individual tracks to different lengths, you’re just making use of the Ch Len feature from that mode, so that a queued pattern change happens after a specified number of steps. That way as soon as you move to the riser pattern, you can then immediately (or soon) queue up the following pattern and it will definitely happen after the time you’ve set.
Example - you’ve got a 16 step pattern, but want the riser to run over 8 cycles of that, i.e. 128 steps. Set the Ch Len to 128, condition the riser trig with “1st” so it only happens once, and your pattern will then run 8 times through as the riser builds, and then move onto the next pattern that you’ve queued.
At which point you’ve got 8 cycles to ad-lib any fun knob twisting during the riser, knowing that your pattern change is all taken care of!
That may be what you were already suggesting, but rather than “slow down” any tracks, this approach lets you just take a straight up copy of the main pattern (e.g. the 16 step one) and not have to mess with any of the steps in that to make it into a 128 step version with the riser and pattern change taken care of.