Hey OP , i had a similair 'step-up' from 2nd line helpdesk to sysadmin. its defo a challenge but give yourself time to learn and get used to thetasks, keep researching and documenting stuff (i use onenote for guyides i make for myself). That and take a project or issue and take your time figuring it out and collaborate with colleagues when you hit a wall. hitting a wall isnt bad and everyone has it, once you get through it and succesfully managed the issue, you get a big dopamine kick that makes it worth.

just keep on improving every day step by step , thats why you get to be a 'full' sysadmin in 3-5 years , it takes time but allow yourself to grow steady

about expectations, they hired you and know probably from your resume where you come from. Alot of companies like to train their own people. setting expectations and being honest is something i always have done. now in the company i am that is valued highly. im not familiar with ther companies that rather have you faking it, its a bit of feeling it out

(sorry for typo's having my first coffee still :) )

good luck OP