PM sent. My suggestion is don’t use the SRT files at all–use the SUP files. These are included in the complete Project Threepio files, but not in the smaller subset included with Despecialized. There are also instructions in the included README file for how to include them in a disc or MKV like you want.

For smaller sizes, I’d recommend looking for the smaller AVCHD encodes (often in the same places as the big MKV encodes)–those are still h.264, but with more compression. But they’re really well done, and they’re from the lossless source. Re-encoding with HEVC from the already-compressed lossy source without a lot of expertise may not end up with a better result.