I'm confused: are you talking about
Bitcoin Classic (BXC), or Bitcoin Core? The former is virtually worthless, so I assume it's the latter.
The problem is that the BTC balance has been reset and now sits at 0. Also, the wallet itself is 8 years behind, whereas it was fully updated before.
It sounds like your software lost access to it's blockchain, probably because you've changed the drive's location.
I tried to move my BTC to a new wallet via the .dat file that I have saved. The new new wallet I tried and installed (on my main hard drive this time) is called Electrum.
Electrum can't access Bitcoin Core's wallet.dat directly. That's probably what causes the UTF8 error.
1. How can I add my BTC to my old Bitcoin Classic wallet so that I can carry out transactions again? When I try file -> open and select my back up .dat file, it says that the payment request file is too large
2. How can I resolve a UTF8 error on Electrum using the file -> open technique I described.
Let's take 2 steps back: first, make a backup of your wallet.dat. After moving your drive's location, your Bitcoin Core installation may have created a new wallet.dat. You'll need your original. Make a backup on a USB stick, then make another backup (on another device).
After this, you have 2 options: either run Bitcoin Core, let it download 300+ GB (this can take a few days depending on your computer), and your balance should be up to date again.
As an alternative, you can export your private keys from Bitcoin Core, and import them into Electrum. This is faster, but involves more risks.
If your wallet is from 2017, you probably own
Forkcoins too. The values in my link are outdated (it's much lower now), but after securing your Bitcoin you might want to look into that too. But first: make sure you know what you're doing so you don't risk losing your bitcoin over this.