Changelog v3.4.0:

#Fixed: setting phantom_spawn.player.blocks.no_solid_blocks_above to false caused the plugin to crash.
#Fixed: reload command caused spawning to stop for all online players.
#Fixed: setting phantom_spawn.player.score.min_score below 72000 (equals 3 vanilla day cycles) did not have any effect.
#Fixed: only the max health of a giant_phantom was modified which missed the actual purpose.

Also please note that while testing the movement speed of a phantom did not change noticeably so changing this attribute may have no effect on the calculations by the vanilla game. Unfortunately in this case there is nothing reasonable to do to change that. However the plugin sets the values correctly, they are just not interpreted by the vanilla game as intended.

Best regards,
Alex_qp

PS: Special thanks to @REjoin for pointing some of the bugs out to me.