This would be an interesting sale but China has 24 Su-35 not 28!
Those 24 were also relatively forced due to friendly state of affairs and warming relations between Russia and China back in around 2015 times when China did try to pull out of the Su-35 deal which was considered and informally agreed between the two around 2010.
Su-35 being the hedge bet against J-11D program and J-20. J-20 became more successful than anticipated and the only one to take over the top tier in PLAAF out of the three paths - J-11D, Su-35, J-20.
J-16 was multirole and while it is better than both Su-35 and J-11D (even if the program finished as it was able) the J-16 is not air superiority focused as J-11D program is.
It was too diplomatically difficult around 2015 for China to renege on the Su-35 agreement with Russia and both discussed a compromise, a very small batch purchase as a gesture. Something Chinese agreed to happily because even though it spends money, at least those Su-35 are useful as even some small means of giving more work to adversary mission planning and strategic planning, it also offered China an insight into the best Russia could offer for 4.5 gen fighter. It is far superior in electronics and performance to Su-30MKI of India's and still working on the same basis of both radar, electronics, weapons, and kinematic performance, only far better in all those domains than the Su-30MKI. For example, their TVC performance and basis is very similar but with Su-35 being slightly better.
So lessons learned, aircraft and its abilities totally evaluated, mission planning distractions for Japanese and others used, it really is not that useful anymore. I think most Chinese military observers and decision making groups would love to sell them to Pakistan if such a sale has a path.
These fighters are not cheap, they are also very expensive to maintain and keep operational. Pakistan would have to go to Russia for parts and services since the engine is 117S and TVC is prone to needing more maintenance. Is it worth it for Pakistan? I'm sure China would be very happy to offload Su-35 but will Pakistan want to pay a fair price for them? I do not think it is in Pakistan's interest to have a small fleet of 24 Su-35 which are going to be extremely complicated for maintenance to keep operationally ready. Pakistan will have to negotiate with Russia and while Russia will not really care if they are sold to Pakistan, will they not care when India pressures them and buys their favor to keep parts delivery and service as slow as possible?