use some TOR browser bundle or something like that
First, this was an I2P link not a Tor .onion link. They are different, non-interoperable anonymity protocols.
Second, using Tor browser directly would be zero layers of sandbox. Running Tor browser inside a VM would be one layer of sandbox.
someone trying to use JavaScript against you, or leverage other browser techniques to leak your IP
Ummm... first the whole point of Tor and I2P both is that nobody knows your IP address. Not even the website operator. Not even the darknet node operators, except the entry node you initially connect to (it kind of has to know your IP address for that to work). And that entry node can see only your IP address -- it has no idea what sites you connect to from there or what data you transmit.
Lastly, if you're taking the trouble to use Tor or I2P in the first place -- turn off javascript. Or even better, just move the slider in Tor Browser to 'Safest' mode (why that isn't the default has been the subject of a religious war, one where my side so far has lost, so you do have to do this manually)