Help "Building an own smeltery block for moditems"

i think that not possible because if you want the block to change texture you either need animated texture or blockstate like change form unlit custom furnace to lit custom furnace and that completely delete the block but i think you can do like check for the number of item and then set new block with the same amount of item texture part done.now the procedure for smelting https://www.youtube.com/watch?v=0vnkebRDZfY please note that you need to change the setting to alter your gui because the template set the get number on slot id 16 you need to change that

also here the picture https://sites.google.com/view/polarcraftwiki/home/tutor

also if you want any help just ask me(thought im in s.e)

You can use the procedure: (replace the block at: <x, y, z> with <block> and keep the state, keep nbt / inventory) to "change" the texture of the block, in fact it will replace the block with another one with the desired texture and with the same function, maintaining the inventory, the status of the block and the nbts tags of the previous block.

And to do things every 20 ticks, just set the tick rate of the blocks to 20, that is, if the code is placed in (Update Tick)

Another thing ... I'm afraid that using your logic the game will get stuck in the While, I would use one (break out of the loop) after doing the action

yes the template get some bug so it crash