As far as I know there are few ways to resolve an unstuck transaction.

1. CPFP (child-pays-for-parent) 2. RBF (replace-by-fee) 2.1 FSS-RBF (first-seen-safe) 2.2 Full-RBF 2.3 Opt-in RBF 

My question is simple:

Regardless of the wallet type, If USER A does a stuck transaction to USER B. How can USER C (myself) resolve unstuck transaction without the need for a private key?

Miners charge X amount to push unstuck transaction to their next block.

Opt-in RBF, is possible only if user opted in to replace in advance.

I find FSS RBF quite expensive for cases with big transactions. If the transaction was originally 3 bitcoin then to create RBF with higher fee would require again 3 bitcoin which is quite expensive.

FULL RBF is not supported by many wallets.