Hi teknonix I had a situation similar as I lost about 2 to 5,000 btc but I still have my wallet file. Is there anyway you can help me get my bitcoin back. I can give you tip . Please say.
If your problem is related to change addresses, I can help you. I am able to recover the lost change addresses in the core wallet.
In fact there is no need to back up the wallet.dat after each transaction. If the change addresses are missing, there is a way to solve it.
Today it is easy to get the change addresses and every other address from your backup, but and it is a big BUT...
...In the old days wallets weren't deterministic. Meaning, it was impossible to derive the next keys from your wallet backup.
When the core needed more addresses, it generated them randomly (not deterministically like now) and if you lose those randomly generated addresses there is no way to generate the same keys again later.
Actually the 8999
BTC, that you mentioned, are the reason that inspired the development of deterministic wallets that we use now. So that the same wouldn't happen to anyone ever again.
PS. The 8999 coins will forever stay in the address 167ZWTT8n6s4ya8cGjqNNQjDwDGY31vmHg