Suppose I would expand my BTC portfolio to 10 or 15 BTC.....how would you store such a big amount of coins?
Ofcourse I would use a hardware wallet, but if I store them all on 1 wallet and I lose the wallet, or the house gets on fire and it burns, then I have lost those coins forever? Or are there still ways to get to the coins somehow in that case?
You should have a back-up of your wallet seed in case of hardware failure. You should never depend solely on one device. Consider keeping a copy of your seed written down on a piece of paper and hidden. Also consider keeping an encrypted copy in another physical location (like a safety deposit box or your parent's house) in case your house burns down.
I'm looking for the safest way to store a big amount of BTC and to minimize the risks of losing a big amount.
I would avoid using closed source wallets like Jaxx or Exodus. I recommend Electrum and Bitcoin Core.
You might consider using multiple security models. An encrypted cold storage wallet.dat here, an encrypted cold storage Electrum wallet there. Multiple copies on different mediums (USB, disc, SSD) in case of hardware failure. Maybe a paper wallet. You never know what vulnerabilities might be discovered later -- best to spread around your risk to different models.