Man, can I relate to this! I tried turning mouse, trackpad, and keyboard on and off, hitting keys at different times during boot or wake up, holding keys down, getting coffee, mentally begging Bluetooth gods, and impatiently waiting for the next macOS update to see if anything improved!A few seconds only? That's great and gives me hope for the next device. Perhaps my Apple keyboard is a less-than-perfect specimen. 30-plus seconds was not unusual. 15 seconds was cause for celebration. The last straw was a delay of just over a minute before I could type my password. Every boot became a game of "how long is it going to take this time?"
The batteries were fresh and my patience was spent.