Getting/sending some information from Telegram at bot startup, after Application build() but before run_polling()?

I’m a bit confused about this because Im new to python-telegram,-bot, and most examples online dont reflect the v20 changes for Updater and asyncio.

The relevant snippet of code is:

def main() -> None:
    persistence_object = PicklePersistence(filepath=persistent_data_file_path)
    application = (
        ApplicationBuilder()
        .token(bot_token)
        .persistence(persistence=persistence_object)
        .post_init(post_init_handler)
        .post_stop(post_stop_handler)
        .build()
        )
    application.run_polling()

async def post_init_handler(application: Application) -> None:
    print("init")
    bot_id=application.bot.bot.id
    # Needed: Code snippet here (details below)
    # It produces a list, which is sent as a raw text private message to a static defined user_id

if __name__ == "__main__":
    main()

The missing code snippet obtains a list of all chats the bot is in (private, group, supergroup), with some key information about each (see below) and then send that info to a specific user_id as a private message. It doesnt enter polling until that’s succeeded. The info it collates and sends is:

(chat id,
 username/title of the chat,
 chat type (private, group etc),
 is the bot the owner of the chat? (if a group/channel),
 bot's admin rights in the chat (if a group/channel and admin)
)

So a typical output sent would be:

123456,@fred99,private,(ignored),(ignored)
872981,@technogeek_chat,supergroup,True,(rights data)
(etc)

I cant figure out the correct way to get and send this data at bot startup.

• CORRECT API CALL/S TO USE?
  All the examples I can find, assume their actions are done reactively within some callback handler as a result of a polled received update, or if done initially, the example uses older Updater/dispatcher techniques not applicable in v20.
  I * think * from the Transition guide to v20 that application.create_task could be the correct answer but if so I dont have a good example to rely on, how I’d use it for this.

• CORRECT PLACEMENT FOR API CALL/S?
  I tried to move the code to between application...build() and before run_polling(), but the assignment bot_id=application.bot.bot.id failed and I couldnt get that to work. It wanted it initialized, I think, but I wasnt sure what was correct, or which was the more correct place of the two for this code snippet, and left it within post_init_handler() which at least * seemed * to work?

• GETTING DATA FOR THE CALL/S?
  I tried to find code to list the chats the bot is in. The only links were to the example "chatmemberbot.py", and that code doesnt actually detect what chats the bot is in. It relies on adding and removing from a manually maintained list based on updates. Which could potentially desync from actual chats it’s in, in some situations (bugs, persist store deleted/corrupted/old store restored, shutdown prior to final flush in the past, …) and the bot would never know its list wasn’t correct.

What’s the correct way to do this, please? Is it possible to have an actual example – simplified if it’s easier – how it should be done, as I’ve scoured the docs and can’t figure it from those?

Help much appreciated, thank you!!