I don't think there's much to worry about as if you're rooted with TWRP, it'll just reboot into TWRP and wait for you to manually apply it, if you can find it on the file system.
I'm currently trying to apply B356 which was recently released. This minor update contains the July 2018 Google security patches. The last time I did this from B351, this is what I did:
Via TWRP, Installed the 2 full OTA zip files for B352 downloaded with firmwarefinder. This replaced TWRP with the stock recovery but did not wipe my data.
Rebooted, and applied B353 via the normal OTA through settings. This installed the B353 full package (2.52GB).
Reinstalled TWRP with fastboot (twrp_BTV-W09_5_greatslon.img)
Booted to TWRP & installed Magisk
Re-enabled the 5GHz wifi radio
However, this doesn't seem to be working with B356 thus far. Installing the Firmwarefinder zip files failed (at least update.zip did, I didn't try the other two) so I installed the FF B352 images mentioned above. However, applying the stock OTA through settings wants to install 3 patches (B353, June 18, July 18) of size 786MB. They downloaded successfully but then it fails to apply during the reboot.
Anyone else have any luck? What did you wind up doing?
Thanks!