Is it possible for a bad actor to work out where my node is located, thus making me a target for physical burglary?
It's very unlikely considering there are about 21K nodes that accept incoming connection[1] and about 87K of all running full nodes[2].
What basic security actions would you recommend to maintain my anonymity please?
Also should I take any action against online attacks?
By default, Ubuntu is secure enough (assuming you're not specifically targeted by someone with tons of resources). Even Ubuntu help page only mention very common advice[3]. If you don't want your IP address known by other full node and any website you visit, VPN should be good enough. But take note some some VPN provider doesn't take privacy seriously.
[1]
https://bitnodes.io/[2]
https://luke.dashjr.org/programs/bitcoin/files/charts/historical.html[3]
https://help.ubuntu.com/stable/ubuntu-help/net-security-tips.html.en