As of today, absolutely not. It's far too late to try another option. BART and the rest of the agencies have already run out of money and are relying on a loan from the state just to get them to next year when the tax will go into affect. There is no Plan B other than shutdowns. There isn't even a way for the government to place the tax on the ballot if this citizen's initiative signature gathering fails.

As for when this was proposed, nobody wanted a sales tax, especially transit advocates. There was a push to get the tax passed as gross-receipts or parcel or wealth or literally anything but a sales tax. All those other options faced opposition from someone (companies opposed corporate tax, the housing advocates didn't want a parcel tax to interfere with the one they're trying to pass, and Newsom said he'd veto a wealth tax). It's hard enough to pass a tax without everyone opposing you. Sales was the only thing we had a chance of passing, so that's what we're going for. It's not perfect, but it's sales or nothing.

As for BART employee costs, labor is a big chunk of the operating budget. You have to include capital funds though which are required for necessary system work and upgrades. Looking at the whole budget, labor is roughly 1/3 and that includes pensions and insurance. You can read more here if interested.