You also need to take light mode and fast mode into consideration. Fast mode requires at least 2 GiB RAM which is not possible in a browser. There’s around a 5x difference between the modes based on my testing. If Syrius is running in fast mode, then the difference in speed becomes 50-100x, assuming your estimation of 10-20x is correct.
I would start with a PoC to prove that RandomX plasma PoW can be computed within a reasonable time in a browser before basing any decisions on that assumption. I doubt it will be fast enough to be a viable option and not worth the additional complexity it introduces.