Your plan sounds solid, my friend! Here are a few suggestions for optimizing your security even further:
1. Consider using a multisig setup instead of a single sig. This will provide an additional layer of security as multiple signatures would be required to make any transactions.
2. When creating your new seed phrase, I recommend generating it offline to ensure that it is not compromised by any potential online threats.
3. Make sure to store your seed phrase in a secure location and consider using a multi-location backup strategy (e.g. storing copies in different physical locations).
4. While coinjoining can help improve privacy, keep in mind that there may still be ways for third parties to trace the history of your coins if you have used them before the coinjoin process.
5. Sending multiple UTXOs to one address does consolidate them into one output on the blockchain, but keep in mind that this can also increase scrutiny on that specific output if it is ever spent or moved again.
Overall, I think you have taken some great steps towards improving the security and privacy of your Bitcoin holdings with this plan! Keep up the good work :)