Hiring systems engineer and/or technology lead? (I will not promote)

We're a seed-stage startup ($4.2M in funding) hiring a systems engineer and/or technology lead. Our team is highly technical. Currently, I am doing "double duty" as CEO and CTO. I'd like to hire a systems engineer and make sure my expectations are properly aligned. We recently made an offer to a senior BME systems engineer (130K, 0.5% equity), and he shared that the equity/salary was too low for him. We have a strong technical team that, given priorities, will execute them well. However, they currently need someone to help set and review priorities and acceptance criteria. The product is in the robotics space, and the team comprises 3 Robotics PhDs (including myself), 1 BME PhD, 1 MechE PhD, 2 robotics MS, and 1 BME MS. The team has currently become organized around people coming to me for high-level guidance and quality checking, which is unsustainable. Our advisory board is also highly technical (MDs/PhDs in this space, billions in exits as executives). What are reasonable equity and salary levels, and how would you approach hiring at this point? Writing this out, it feels like our team has enough expertise that someone internal should be open to "rising to management". However, it currently feels like most prefer to remain as ICs.