After that we've used the playtex liners. They're LDPE (polyethylene) and not known to leach any chemicals known or suspected of causing cancer or disrupting hormones. The nipples are 100% silicone, with the only cautions coming from cooking in silicone (primarily baking) far exceeding the temperatures you'll ever get warming a bottle. Make sure any glass bottles have silicone nipples too (some brands don't).
The reason to use the disposable liners is basically: no concerns about BPA, less hassle, you can warm the milk faster, and you only have to sterilize the nipples (most important in the first few months to prevent thrush, which can stop them drinking).
Using stainless steel is risky due to manganese, which means they're either coated with polycarbonate meaning BPA, or PET. So you've got a stainless steel coated polyethylene liner... kind of redundant.
We used to float the liners in a Thermos full of hot tap water, it was usually at temp within 5 minutes (you can do the stir and shake in emergencies, and you get it warmed in about a minute).
Good luck, don't panic! You need to conserve that vital energy for sleep. My son is almost 15 months. I've been so long without sleep, that the rare occasion I get 8 hours or more I think my body just assumes I'm dead and starts breaking down.