When the correct answer is “bitcoin“, what is the question? I genuinely, genuinely, don’t see any task for which it is the best solution — not even for anonymous online payments or secret ways to store wealth.

"what is the most well tested and carefully developed payment system today that allows me to have complete control of my funds without having to trust any third party?"

I see your suggestion and raise you “precious metals”

a) Precious metals are not portable enough. With Bitcoin someone can transfer any amount of value within a few minutes to the other side of the globe (and no-one can stop you from doing so).

b) Precious metals are too reliant on physical security. Someone can break into your house much easier than into i.e. your hardware wallet.

c) Besides these, precious metals are not a payment system, much less a programmable one. You can't code for example a logic that requires multiple untrusted parties to sign a transaction for it to complete.