Keychain migration blocked

Are there any reported solutions yet to TM/Migration Assistant failing to transfer keychains in encrypted / unencrypted environments? On migrating to a new MBP I had an encrypted machine and TM, the original keychains including safari do not migrate. AppleCare has variously instructed me to unencrypt/delete my TM and manually copy them. They have escalated it to engineering level without success and 12+ senior advisors. The last stated that the manual has ambiguous instructions which are sufficiently vague when transferring manually the keychain to prevent a procedure being followed. All through the SA keep advising to use icloud.

I am concerned that Apple aware but is not prioritising fixing this using High Sierra as previous migrations using earlier OS have been flawless. The SA were unable to say what other files are not migrated. I am not clear if TM even backups up keychains. I am researching third party software and departing the apple/safari/keychain/TM storage system except for core system passwords. The amount of time lost in business is a huge factor in this decision followed by a lack of trust in the nature of the backup.