Buy/Sell crypto for family members? Easiest Method to assist/custody?

I have a family member who is a bit too old to handle all the security protocols. What's the best way to custody someone else's crypto account without any tax consequences? They are very savvy about all the scam emails, links , private wallet info, etc. , but a bit overwhelmed by all the security, 2FA, etc..

I want to get them invested a small amount (through donations).

If I buy an asset and move it to "their" wallet (in my possession), a month later, can I "sell" it to them at whatever price I choose (i.e. the price I paid)? or must the price reflect the timestamp the transfer to the wallet is made? In other words I buy 10 CHAINLINK today and I don't transfer it for a month. Can I "sell" the crypto to them at the price I bought it at (at the point of withdrawing to their private wallet)? My plan was to manage the asset in a hardware wallet until it's time to sell, then transfer it to their Exchange account and just have them sell it. And then provide them with a receipt for cost basis. Looking for lower cap alts for a small investment, not top 10.

Also, is this even legal to "sell crypto for money" or does this make me a "broker/dealer" or something to that effect? I know you can sell crypto for an asset, but I would not be surprised if this was different.

What kinds of tax scenarios does this bring up? I'd prefer to not blur the lines of custody and ownership in any way, to keep it simple, and not have any "3rd party" activity in either account. I don't want any entanglement. I only buy/sell on exchanges, and store in wallet, no outside transactions. The person will benefit from their long term cap gain limits vs me just keeping it in my account until cashing out.
The other concern is if they open a Coinbase account, say a year from now, and don't deposit any fiat, but only deposit crypto and immediately cash out, will that cause some kind of security block on their account? What's the best way to custody someone else's crypto account without any tax consequences? Thanks.