He doesn't have any nice specific blockstring routines or even nice blockstring enders. It isn't like, say, Nanaya where you use 2b when you're far, 2c and 5c to move forward, 5b as a clashy move and 236a/b pressure to move forward again. Generally you want your Kouma blockstrings to be short and frequent; you want them to be constantly afraid of you dashing in and grabbing you, whether you decide to on a certain iteration or not.
There are some other tricks to blockstrings and pressure:
-5a will whiff short crouchers
-5{c} looks very much like 214b
-2[c] moves you really far forward if they don't have any good far pokes, it's also +3
-5[c] will catch people who try and jump out of it
-There's always 236b 22c (on block) if you need it
-Also, 236b is very unsafe (like -12 or something) on block. However, the super-cancel window is really big, so if they are punishing it you can cancel into 214c really late to grab their poke. If you get this in their head, you can make 236b very powerful because it moves you forward, has a hit of superarmor, on hit leads into knockdown, and gives you this safety net on block.
-Tick throwing after dashing up is fine too.
-The way you prevent people from jumping out of your grab attempts is to 623a/b them out of the air. This forces them, when you approach, to either sit on the ground and guess what you're going to do, backdash (putting them closer to the corner which is great for you), or eat a DP and be put into the corner. However, watch out when people have high priority aerial moves or aerial projectiles, because they can easily start baiting your DPs. A lot of Kouma's bad matchups are with characters who can do this. You really just have to find an alternative depending on what exactly they're doing.
A good Kouma will not throw out pokes that he expects the opponent to block. A good Kouma will expect the opponent to block and disguise his grab attempt to seal the deal.