The problem with the migration assistant is primarily with incompatible applications being brought over to the M1 Mac.
As long as all you are doing is documents in migration, you should be fine.
If you need specific settings, those may need to be rebuilt on the M1 Mac, but use the old Mac as a reference.
Keychain Access in Applications -> Utilities allows you to look up some of your passwords when you doing the rebuild.
Some with the browser preferences, many passwords are stored there.
You can export browser bookmarks to HTML from most browsers, and then import them after migration.
A late 2009 iMac can migrate over ethernet to a USB-ethernet adapter with the M1 Mac, or even establish file sharing that way for the fastest link.
Also possible is Firewire to Thunderbolt -2 and the link Thunderbolt 2 to 3 adapters from Apple. But that may have some lag trying to do a data transfer since adapting technologies frequently can slow them down.
The Superduper backup can also be used as a migration point to your new Mac. Just make sure that you use 100W pass through USB-C adapter that powered the adapter before linking the hard drive, if the hard drive was USB-3 or earlier.
See https://www.roaringapps.com for application compatibility with Big Sur a before buying an M1 Mac.
M1 Macs don't support Boot Camp. If you need Windows support, get an older Intel based Mac.