UPBO should be enabled as it's mandatorily required by BT Openreach's SIN 498.
As for power output, this will dynamically lower itself based on the target SNRM and your current SNRM. I can see your SNRM is higher than the standard 6dB target SNRM so as such the power output is reduced in order to reduce crosstalk for other neighbouring lines. As it gets closer to the target SNRM the modem and DSLAM will negotiate a higher power output accordingly on their next synchronisation.
With the DSL-AC68U I have managed to override the power output, as it's a very powerful modem it seems it can do a lot of things that isn't expected or normally provided for that matter. I wouldn't recommend fiddling with that type of stuff though.
As for your speed cap, your DSLAM/circuit must be G.INP enabled, so enabling G.INP will be beneficial to you most likely. Over time DLM should hopefully increase your speed banding (cap). It it doesn't do so after a few weeks of uptime then you might want to consider contacting your ISP if your speed is below the estimate.