Which is the most convenient (and safe) way to obtain cryptos with FIAT? (excluding mining)
Every method has pro's and con's i'm afraid... an usually it's a tradeoff between trust, physical security and how easy the system is.
You could go for non custodial exchanges:
https://bisq.wiki/Frequently_asked_questions#Is_Bisq_safe.3FThe risks on these exchanges are fairly limited (but they still exist!!!) and they're relatively easy to use (but they do have a learning curve)
or, you could go for centralised exchanges (like binance), which require KYC and have loads of rules and regulations... but if you use them to exchange fiat for crypto and immediately withdraw to a safe wallet, they're relatively easy and secure to use (but you offer up your privacy)
or, maybe use a bitcoin ATM? Easy to use, safe if you use a trustworthy atm, but usually quite expensive and hard to find (and they require you to physically go to the ATM, which might threaten your physical security).
or, a face to face trade, which gives you pretty good guarantees that you'll receive your crypto (if you use decent opsec), but there's a risk for your physical security.
Like i said, i don't think there's an option that's super easy to use, super safe (so you don't get scammed), super safe (for your physical security), cheap, private,... It's always a tradeoff.
For a beginner, i'd probably tell you to use a trusted, big, regulated centralized exchange... Do your homework, follow the KYC procedure, read the rules,... and offer up your privacy... This way you can buy crypto in a relatively safe way with a minimal learning curve, but it will cost you a little bit of privacy.