Negotiating Uber offer

Got a senior 5a offer with 180k+ base and 16k+ shares (worth $650k+ if $40 a share). Hr told me that base and equity does not have much upside, but maybe can throw in some sign on bonus. Question 1: is it true that equity can’t go up much? Is it true that recently Uber has changed and don’t negotiate too much, don’t compete with google/fb etc? hr also said that equity grant is tied to level, once level is decided equity is determined. Question 2: the offer does not have signon bonus but hr hinted that it’s the most negotiable. Especially if first year pay at Uber is less than the pay at my current company. If I stay at current company I expect 270k annual pay (base + bonus + vested stock). If I go to Uber my first year pay is 180k+, since any annual bonus or stock vesting will be after one year. If I ask for 90k sign on bonus, will it sound very unreasonable? I’ve seen 100K+ sign on bonus before (from fb or amazon), but not sure what’s the usual range at Uber. my friends at Uber say Uber only started to offer sign on bonus recently, so it’s hard to find what is a good range. Thanks for the help!