thankyou for the additional info...
while mounts on the same disk are technically not unsupported... ( and are different from 'overlay' mounts )... they are something i'd only advise for seriously skilled users...
primarily due to the behavior of sysupgrade when additional partitions are present...
for more space... the advised option is to use a usb drive and a non-overlay mount in /etc/config/fstab...
this is what I have (can be done graphically in luci)
config mount
option target '/usbstick'
option uuid 'aa8fca3f-7077-4f41-a289-ca04fc22470d'
option enabled '1'
(ext4 but fat32 / vfat maybe f2fs should work also... ntfs has some quirks of it's own)
these mounts do not interfere with sysupgrade and are immediately accessible on the new system...
if the card ever needs to be re-flashed these mounts are easily recreated or found again from a backup restore...