Here's my paranoid guide for keeping your wallet safe & secure:
I have printed the seed
That might already have screwed you up depending on how your printer was connected to your PC. Writing down the seed is the preferred way imo; make sure to use a solid desk pad to avoid imprinting / engraving what you write to other materials below your paper wallet :) In addition to that you can create a steel "wallet" containing your seed, see https://cryptosteel.com/
I'm using a read-only wallet on a computer connected to the Internet
That means this computer holds your private view key making it possible for hackers to see the amounts you're sending and receiving. Not too unlikely given that the computer is connected to the Internet. May I ask what the reason for this Internet-connected computer is? Why does it need to have view access to your wallet?
I keep on a USB key the full wallet as cold storage
It might be worth creating an encrypted container on the USB stick, e.g. with TrueCrypt 7.1a or VeraCrypt. In addition to that, you can encrypt the wallet with GPG to have another fence in case TrueCrypt or VeraCrypt was backdoored. Make sure to only use these tools from a computer which will never be connected to networks, e.g. the Internet. Also you should have multiple backups of that USB stick at different places, preferably in different countries in a fire and water proof safe.
a dedicated local server for the Monero node
Just to be sure this node (even if it's downloading the blockchain only) should do so through Tor via torsocks.
When creating your wallet, make sure to use a computer which will never be connected to any networks. Remove its harddrive and WiFi antenna. Make sure to use a good random number generator, how to ensure that? I don't know. Use an open source OS, e.g. Debian or Tails. Only download software from trustworthy sources and verify its integrity.
I recommend to split your balance across multiple wallets; even if you somehow lose access to one (maybe due to hackers, robbers, etc.) you have still access to your other funds.