But a lot of this is too troublesome or adds too many hoops to jump through than is practical for many people.
If it’s risk of customer funds mismanagement, coinbase is just not in the same galaxy as the folks who have blown customer trust.
If it’s securing funds, to prevent hacks, CB offers cold storage. Lumps of crypto can be transferred out of their control as needed to manage risk.
> But nobody has addressed that market.
Au contraire! You've correctly identified an important market! There is both existing work and ongoing development that tries to satisfy exactly what you've asked about (and doesn't involve burying etched steel plates).
Encumbering your bitcoin with the requirement for multiple, M of N threshold signatures ("multisig") is an important way to protect large amounts. Companies like Unchained Capital [0] provide a service wherein the user holds two keys and the company holds 1 key in a 2-of-3 multisig setup; if the company key is needed, video authentication and other procedures are required.
Other, non-company-assisted multisig setups use schemes such as you propose; one example is the Nunchuk wallet [1] which allows you to sign multisig transactions on your own, or request signature(s) from a key held by a family member or friend, passing the PSBT (partially signed Bitcoin transaction) over a secure communication channel.
Finally, two great examples of physical devices to protect your Bitcoin are (1) the Tapsigner [2], which is an NFC-enabled smartcard holding your secp256k1 private key that does on-card signatures; and (2) Jack Dorsey's Block (formerly Square) is developing a hardware wallet that integrates with your smartphone [3] -- one neat innovation here is that policies can be set such that the user may spend small amounts of funds with the phone only; but larger transactions require a thumbprint or pin on a physical device.
You also mentioned time delays -- this is also supported by Bitcoin script; advancements such as miniscript [4] allow you to express complex spending conditions in a tree-like way.
[0] https://www.unchained.com/ [1] https://nunchuk.io/ [2] https://tapsigner.com/ [3] https://wallet.build/ [4] https://miniscript.fun/