I watched some guys videos and they used glowstone and slabs. Their farms works great and mine not so well ahah
Pretty sure glowstone won't work, because the game considers it a transparent block. Jack-O'Lanterns are a light source that is a non-transparent, spawn compatible block (I've used them in some of my slime farms). Upper slabs should work (lower won't).
My old golem-on-cactus slime farm design still works in 1.14.1 (imported world from previous version).
Put an iron golem on top of a block, on top of a cactus in a slime chunk (in this example, 2 diagonally adjacent slime chunks) and surround the cactus with hoppers (preferably 2 rings of hoppers). Slimes will spawn and try to attack the golem, bump up against the cactus and die, dropping their slime balls onto the hoppers (FYI: Hoppers are valid spawn blocks).
Use fences to keep the golem in place, and make sure the golem is visible from everywhere in the slime chunk. Point hoppers into a collection chest or system. Area prep is key to a productive farm (eliminate other spawn locations near the slime farm).
Another example from my Broken Laptop SMP server (2 horizontally adjacent slime chunks).