The first question you should ask is - who is your enemy? And then adjust your opsec appropriately.
Are you perhaps worries about...
- someone logging into your email and reading sensitive data
- someone sending you (or your colleagues) malicious links to spyware, ransomware etc.
- your email being abused for something like social engineering or CEO fraud
- government subpoeaning the email provider and getting your data
- reliability and accessibility of the service in all circumstances
Your questions
1) probably everything is better than SMS (physical token, Google Authenticator, bank app, Authy)
2) there are some desktop open-source ones if︀ you are paranoid
3) you can never be sure so use HTTPS always on and︁ also including DNS over TLS which can be biggest vulnerability. For VPN, always use anonymous︂ account details and pay anonymously. If you don't trust VPN, use Tor.
Unfortunately some countries︃ (think Kazakhstan, UAE...) possibly have access to certificate authorities so they can fake your connection︄ destination and do MITM attack even over HTTPS without you noticing. This is however very︅ high profile stuff, e.g. if you are a disident or enemy of the state.
4) use strong password and don't name your hotspot with a name that is connected to︆ you.
If you are paranoid, read e.g. the book from Edward Snowden, in one chapter︇ he writes about how he used the internet when his enemy was the NSA and︈ he couldn't afford to make a mistake.