You can tell your employer you don’t want to pay for the employer subsidized plan, but then you lose access to the employer subsidy and ability to pay premiums with pre tax income.
A silver plan an employer subsidizes is similar to a silver plan from healthcare.gov (expect plan to pay 70% and insured to pay 30%).
They are better than nothing if you qualify for a subsidy, but if you don't and you live in a HCOL area (which the subsidies are not adjusted for) you are pretty much screwed.
The MCOs are not carving out different networks for specific employers. You buy a bronze PPO from United Health directly, or your employer does, it’s going to be all the same providers. If you get gold, then the deductibles/copay will be less.
The networks used to be different when HMOs were in vogue, but if your employer is only giving you an HMO option, you need to find a different employer. PPOs are the only sensible option (except maybe Kaiser on the west coast, but even they sell PPOs, just costs more to see a non Kaiser provider).