Ian Hi Ian, It is not that difficult. The wireless transmitter is a separate electrical system from the CPU and RAM. So more CPU power has no effect on the wireless power, just as much as a stronger CPU isn't capable of making the LED burn brighter, everything else remaining the same!
But one question remains unanswered: Why is the max. number of connected wireless clients go down. There were initial reports of 20+. The OP mentions 17-19. My (only) test about a year ago gave me 14. You are talking of 7. (I am interested only in the built in wireless transmitter. It is clear to me, that additional hardware changes that number.) I thought the built-in wireless chip remained the same since Pi 3 Model B. If I'm not mistaken, it was the same even in their previous wi-fi adapter for USB. Then the number should not change, definitely should not go down. Is the antenna, yes there is a rudimentary antenna etched on to the PCB, getting changed, or there is bug in the Linux driver after all. (There was a discussion somewhere in the Internet to the latter.) From a practical standpoint it is easy to test: get a Pi 3 B and a 4 B, also two versions of MoodleBox and try all the combinations.
BTW, if you depend on solar panels, I would seriously look at Pi 3 B - not even 3 B+. I wonder, for MoodleBox serving a dozen of wireless devices, they make a difference. But in power consumption they do: 3 B+ needs double the power of a 3 B.