This appears to be a known problem, discussed a few times in this forum e.g. Wifi and Bluetooth Have Never Worked - #35 by choboDOC and further link from there.
I don’t know the details of this problem but I wonder whether the only full workaround is to upgrade the WiFi card from Redpine to SparkLAN, which may or may not be something that you would contemplate doing.
I would probably use ping -s NNN ... with progressively higher packet sizes to see whether the problem is not the type of traffic but instead the packet size.
It may help to clarify though what you are pinging and which computer is doing the ping. More generally, other questions relating to a hotspot configuration are confusing because it is not crystal clear to the casual reader which computer a comment relates to (since most of the terminology and most of the commands will apply equally to the client big computer - desktop or laptop - and the Librem 5).
What about tether via WiFi? I haven’t done a lot of tethering but I think that’s how I have done it.