The biggest risks for the heirs not being able to access the coins (as I see it) is either them not being able to access the timelocked transactions, or losing access to the address which the timelocked transactions send all your coins to. Probably the most efficient way to mitigate these risks would be to combine the methods above, and store a copy of your timelocked transaction in your safe as well as handing them a copy to store, so there is not a single point of failure for the timelocked transaction. This would obviously necessitate the "spend an input to invalidate the transaction" method for replacing the timelocked transactions. And while you are at it, they could hand you a copy of the seed phrase or private key for the target address to back up for them (assuming they don't use that wallet for anything else) in a location they know about, although you shouldn't keep this in the same place as the timelocked transactions for obvious reasons.

Other risks would be you forgetting to invalidate and replace the timelocked transactions, or messing up and creating an incorrect timelock without realizing it.