While the current workaround for political parties is through EditESF, by copying faction arrays, I remembered a similar issue during Rome II I managed to fix. So I tested it, and it works for political parties as well. So, the issue is that political parties are not implemented into the startpos file.
All you basically need to do is have the following files at the following locations:
Step 1. The following files need to be in: Total War Attila\assembly_kit\raw_data\EmpireDesignData
political_parties.xml
faction_political_parties_junctions.xml
political_parties_power_effect_bundles.xml
Step 1 screenshot:
Step 2. The following files need to be in: Total War Attila\assembly_kit\raw_data\EmpireDesignData\campaigns\main_attila
political_parties.xml
faction_political_parties_junctions.xml
political_parties_power_effect_bundles.xml
Step 2 screenshot:
**Step 3. The following files need to be in: Total War Attila\assembly_kit\raw_data\EmpireDesignData\campaigns\main_attila
'political_parties' in Total War Attila\assembly_kit\raw_data\EmpireDesignData\campaigns\main_attila\political_parties_tables
'faction_political_parties_junctions' in Total War Attila\assembly_kit\raw_data\EmpireDesignData\campaigns\main_attila\faction_political_parties_junctions_tables
'political_parties_power_effect_bundles' in Total War Attila\assembly_kit\raw_data\EmpireDesignData\campaigns\main_attila\political_parties_power_effect_bundles_tables
Step 3 screenshot:
**The above 3 files in step 3 are the files you can convert in BOB. Basically the raw_data > db files, converted into working_data, copy and pasted in the above locations.
Make sure all the files in every one of these locations are up to date. Now build the startpos through BOB. You will need to manually replace them again as you're changing them in TWeak.
Conclusion
I am pretty sure other regular DB tables' content can be baked into the startpos the same way. From custom buildings to custom names, they only need to be present in the above location.
Random Tip
If your startpos ever fails to build, make sure that all mods are disabled in the Attila launcher. While the startpos builder may ignore the Launcher as Attila starts, it'll still use those mods upon startup, if they're activated. When that happens and it conflicts with your startpos data, your startpos will not complete.