you can get 3.7k with 5B starter on crouchers using the 6C route.
if you haven't used 5D previously in the same combo after you use BE, you can go into either CID/DID (depending on the proration) into straight, then once you land, run the other way and perform 3C (if done correctly you should still be facing the corner). from then on you can apply 3C oki.
against ragna, jin, noel, taokaka, bang, carl, makoto, and platinum, you can use what we call TK GH. this is performed by 2147, then pressing B right when ragna jumps off the ground. if done correctly you should be able to connect with 5B afterwards.
for pressure, once you get 6D on the opponent, you can either cancel into j.D for an absolutely safe blockstring, which you can then apply more mixup or pressure, or you can whiff j.D by performing it really close to the ground, then using 2B afterwards. in combos it's generally used in the corner, although against some characters you can do something like 6C > dc 6B > 6D > late j.D > dash 5D(1) > 214D > dash 5C midscreen. 6D in BK combos is not necessary, but it's without a doubt the most used move in an optimal combo route.
ragna's back throw is pretty much the same as forward throw, minus the fact that back throw puts the opponent behind ragna instead. use the backthrow to put the opponent closer to the corner should you yourself be close to the corner.