For me, the definition of "secure" depends on how paranoid I'd want to be.
Tails is non-
free. In order to be compatible with as many devices and hardware components as possible, the team behind Tails had to add non-free blobs and firmware. Otherwise, the experience on less-known or less-supported hardware would be crappy.
Hence, if you want something that is secure as in an OS that is
FOSS, you could get yourself a free copy of Debian and install it on an old PC.
Either use it as an online or an offline wallet together with a verified
Electrum install. If you have an old PC laying around, you could actually turn it into a dedicated airgapped wallet for Bitcoin.
Live USBs and Tails are good for when you're on-the-go, but live USBs willl likely require you to reinstall Electrum every single time you boot them. Moreover, if you want to use Tor as well, you will have to go through extra steps
every boot. Since Tails comes with Electrum preinstalled and system-wide Tor, it's way more convenient.
Another option is installing
Qubes OS, although it's not a newbie-friendly OS to run. You can run multiple operating systems in Virtual Machines at the same time and make two Qubes (VMs) for both offline and online wallets.
If you go for the USB setup, be careful where you're going to plug it since it could still be affected by an infected PC.