2) directly connect your raspberry PI with your PC and change the data directory of your PC's wallet to the data dir of the full node on the raspberry pi.
This makes it not necessary to have the whole blockchain downloaded on your desktop PC.
As far as i remember, this only works if the daemon on the rPi is no longer running... You can't run a daemon on the rPi and while it's running use the blockchain stored on the rPi for running an other daemon or a QT client.
OP... If you're comfortable using the cli, there is the option of just using json-rpc commands, altough i would recommand setting up an stunnel between your local machine and the rPi for encryption purposes
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To do this, first setup stunnel, then edit your bitcoin.conf to allow connections from your client machine and setup a strong user/pwd , then install bitcoin-cli on your client. Offcourse, you'll need to add the following parameters to each command you send: -rpcconnect=<ip>, -rpcport=<port>, -rpcuser=<user>, -rpcpassword=<pw>. You can also looking into rpcauth instead of rpcuser and rpcpassword...
I've had the same question a long time ago, and as far as i know, that's about the only way to do this using bitcoin core... I didn't find a way to run bitcoin-qt by connecting to the daemon on a secundary machine (and i did search for a sollution for quite a while)
