Would using Tor over a VPN be a good idea for crypto trading?
What kind of tl crypto trading? On centralized exchanges and wallets, not advisable, some even block any VPN or Tor and only allow IP address from service providers to be able to make use of their exchange. On decentralized exchange, VPN is enough while only Tor can maximize the anonymity.
I know Linux is generally (not 100%) immune to malware
Pmalek has given you enough answers to that just to avoid repetition. Also you can have anti malware and frequently updating it, making use of ads blockers like Unlock in addition. If you avoid malware, your device will not be infected. About keyloggers, even bitcoin address will change to hackers address either when you copied and want to paste it to a a sender or while about to send to a receiver, checking and rechecking the address before sending can help and bring your notice to it, but avoiding malware generally will make this not even possible.
But the topic is security; and there it's simplest and safest to have a dedicated device for holding your keys.
Why don't he just hold the seed phrase instead of the keys when we are no more in the nondeterministic (random) wallets era and now in deterministic (seeded) wallets era. This can still make this simple by backing up the seed phrase (which can generate the keys and addresses) laminated paper or metallic sheet.
Why do you think using the TOR version is safer? Couldn't that be a great honeypot as well? Someone creates a TOR-only blockchain explorer that logs addresses. Your IP is protected, that's true. Unless the TOR server you are connected to is also malicious.
There is nothing better than runing own node and have the privacy we need, but due to high ever growing bytes (over 430 gigabytes now) required to run full node and discouraging apect of the prunned node, some people do not just have option than to make use of the SPV wallets, while still just try all possible means to have privacy, but the way to enhance privacy starts from running full node.