A guy inherits 21 BTC as a collection of numerous legacy addresses on an obsolete USB hardware wallet with no screen. The wallet is no longer supported but still works with the original Windows software I/F. The coins are accessible. He also has the private key (not seed words) both in written form and as .pdf on microSD.

He wants to send the coins to a modern HW wallet to secure them properly asap. This will be for long-term cold storage, not for spending anytime soon.

Should he:
Move them on-chain (to Segwit or bech32 addresses)?
Lump sum transfer or send in smaller (what size) increments?
Restore the private key to a new HW wallet?
Spread over more than one new HW wallet? Which wallet/s?

Best practices in this scenario? Practical suggestions? Hints, tips, and pitfall warnings?
What would YOU do to best protect this Bitcoin?