Assuming it's the same as the original PiP (which I only drove for a day), the HV battery gauge stays like that while it has enough charge to work as a 'proper' EV. Once it gets to the point it can only work as a Hybrid, that battery changes to the 8 bars we're used to (unless you'd only drive Gen 1 Prius, which had 4 bars).
Although the PiP only has one (larger) HV battery, the software treats it as if there is one for HV operation and another for EV. I believe (from what some owners have written) if you go down a long hill just after the level drops below the threshold, enough regen can put it back into the EV range and the gauge briefly switches back to the solid display.
I'm surprised by your comments on the HUD, as all the Gen 3 & 4 Prius I've driven with HUD have been perfectly clear, and I think a major safety aid. Maybe that one had a poorly positioned windscreen.
My original (2000) Yaris had digital instruments in the centre of the dash, as has every Prius (all 4 Gens) I've driven since, and personally I like it. The rationale is that the closer the instruments the more eye muscle needed to refocus between distant and dash. It is said to reduce fatigue on longer journeys and I believe it based on the 300,000+ miles in such cars over the last 18 years. Apparently, as your focus gets closer the extra work the eye has to do increases more significantly, so moving the focus from about ½ metre to 1 metre reduces the eye effort more than you might think.
I find the radar cruise control helps no end keeping within speed limits, and for 20 zones I switch to the speed limiter, which quickly becomes instinctive and is very effective.