Hi and thankyou for your lovely message! I’ve been using UHU all purpose (black and yellow) It dries hard after 48hours which is ideal for slippery fibers and easily cracked heads but if you boil the head it’ll turn white and rubbery and still hold the hair tight even though the scalp can be squished. Lots of people use fabri-tac or other non water soluble glues because you want to be able to wash a doll without it falling apart. Same with glue gun glue: a hair iron or boil wash can turn the glue liquid!
Just make sure to get glue all around the inside of the head and dry upside down and you should be fine ^_^
Note: not the UHU green because UHU green has no solvents and takes too long to dry or can seep through as it’s a lot more liquid.