I have been in standalone roles in similar sized companies in an interim capacity and I can see you have lots of great advice below.
I would highly recommend starting with reviewing your role and what it aims to provide to the company or at least setting out the stall of HR.
Consider where can you add the most value?
What are the burning, critical issues and risks for the company right now and what do they need for the future?
What's in place to help you to help others? So what can you automate and maximise and what do you need to leave for now. I'd add the things you want to do on a RAID log (risks, issues, actions and decisions) so you can track them and get buy in from the senior stakeholders for the future.
Say no to things. You cannot be across everything or do everything so focusing on what the HR mandate is and sticking to it is critical.
My last client brought in an external partner to do the basis - like reviewing all the policies. Not ideal but a short term fix to issues that were dragging them back and sucking time that could be better spent on other things.
My final recommendation would be to network and gain support from other HR people. Find a supportive, safe space and mine the wisdom of the crowd and get support for being solo. It's a tough gig so we all need to find our squad.
Good luck.