I don't know anything about battery building but your point about cell spacing and thermal runway is mentioned in Ron's series on DIY battery building
https://www.electricbike.com/introduction-battery-design-1/
Bearing in mind I have no knowledge, but I like the concept of the Vruzend DIY kit connectors, they assemble like Lego without welding, seem to leave some space between the cells,
support individual cell fusing, and I just think its cool you could swap out an individual bad cell relatively easily. However others on this forum have
described issues with the contacts on this type of battery.