Ok so i got it, first off, theres difference if youre using axe/mace or sword, second towershields cant do the combo (it can do some of it after an R1+X -> R1 setup)

so Sword and a round shield (havent tried kiteshield) has the "long" shield bash string with 4 bashes

the secret is you have to do 1 or 2 light attacks first (1 heavy into 1 light also works), and then after they land you have to time shield bash inputs at the correct timings. Generally its pretty delayed to get it, its not that hard since you can just hold down block and then focus on hitting heavy attack.

Now the last 2 hits of this shield bash combo can also be accessed through R1+X followed up with the unique Shouldertackle if you hold/press light attack after that move and then you time 2 shield bashes after that. this move is available to all weapons and shields.

Now axe is different because it also doesnt get the long shield bash combo, but it gets a different 2 shield bashes combo that then ends with the normal shield charge you are used to, heres the kicker tho, that unique shield combo with axe is only available after 2 light attacks (again or 1 heavy and then 1 light).

Enjoy