I was thinking about installing Tails OS with a strong password for encrypted persistence, than inside persistence storage within TailsOS setup KeepassXC database with a really strong password and Yubikey as additional security.
TailsOS would off course be completely offline each time is being used and USB memory stick would be well hidden. Would it than be safe or any safer to store 20-words secret phrase and pass phrase for Trezor hidden wallet? 
I think it depends on how much your crypto is worth, vs how much your bank account(s), stock market trading account(s), etc. are worth. What I have in traditional assets is worth greater than an order of magnitude more than my crypto. I keep all my bank and trading account passwords in my password manager. I have a very strong passphrase, and trust it to store that data. Even so, I keep my crypto on a hardware wallet and my seed phrase engraved on steel plates. Kind of doesnât make sense in a way!
Just remember - you donât want to rely on just a single USB stick. You need a backup. USB sticks donât survive fires - and if theyâre kept in âcoldâ storage, they need to be plugged in every six months, as they do lose âchargeâ over time - the bits need to be refreshed by plugging it in for just a few moments.
Assuming you uphold all those conditions, there is very little practical difference between this setup and, say, a Trezor Keep Metal.
Question is, does your setup have any advantage at all? Itâs still effectively âphysically storedâ seed, with a lot of extra steps.