I’m kind of with Meta4 in suggesting the P4 as i fly the Inspire 1 myself and am aware of the short flight time. Luckily my maps where i have to use the Inspire are fairly small so this worked alright so far. Though for some areas/flights i can use my Mavic 2 and it’s great not having to worry about the time too much or having to restart after changing batteries in our environment with little options to land - a lot of metal confuses the compass hence cannot use mavic due to lack of ATTI.
To your actual camera question, I fly at 30m and get usually around 1.2-1.3cm/px with x3. The most critical thing I learnt is to not use automatic camera setting on the X3, as too often I got blurry photos (in saying this I have not tried the new feature setting for ‘low light’ - this might help if it’s supported to Inspire). When increasing flight height to 50m, also the new Mavic 2 Zoom is losing the quality and it goes up to around 1.6-1.9cm/px (at 30m Mavic2Zoom gets on same flight 1cm/px).
I’m guessing, and someone with a P4 could chime in here, but with a higher pixel camera on P4 (20 vs 12mega pixel in mavics and inspire with X3) you should also see better resolution in the map. So by upgrading to the x5 you might still not get the results in higher altitudes - if this is indeed related to the camera pixels and quality or also the sensor size???
Also, DD seems to have changed the suggested height for new maps from 75m and now suggests for me to not fly higher than 35m. Though speaking of flight time again, if the area is not to huge, maybe 30-35m height might not be too bad of an idea especially if you require the quality you are after?