My PhD Supervisor is founding a startup based on my thesis work and wants me to be first hire. Unsure if want to join
You may find you are an initial ideas person, not someone who enjoys taking an idea through into practice and commercial success. If so, you need to think in terms of getting money for your ideas, rather than for your follow-through work.
Depending on what contracts you have signed and other legalities, it may be too late to get money from your current idea, but look into it. If the devices are not patented, see if they should be, with you as an inventor. Depending on how much he contributed, your supervisor may also be an inventor.
Your objective should be to be compensated for allowing the new company to use your prior work, for example by granting you equity in exchange for a license. Most start-ups fail, so your equity needs to be high enough to allow for that. The net present value of a company that would be worth 20M in a few years if successful may be 2M, assuming 10% probability of success. 2%, even ignoring the job issue, is not enough if the technical basis depends on your thesis work.
Being underpaid by 30K for at least two years would cost you 60K, and it does not sound like you would take the job for fun. Committing to 2 years should result in higher pay to compensate for loss of flexibility. In any case, negotiate separately on permission to use your devices, and pay/benefits for any job offer.