BREAK the sale into 2 parts, "as if" they were two different sales.. That was the first sentence of my answer: "There's no problem at all with reporting that sale as 200 shares and 300 shares, with the same date of course and the proceeds split properly." There's no need to slavishly follow the reporting on the 1099-B as long as you report your income correctly.
You should be able to edit the trade you have worked on to "boil it down" to the 300 share portion, all ESPP.
There's no need to use the RSU guide me step by step process IF you know your basis. The only reason I suggested using the ESPP guide me interview for the 300 shares is that the sale of those shares can create compensation (which gets added to the "out of pocket" basis) and the ESPP will handle that nicely, posting the compensation (if any) to the correct line of Form 1040 and adding that to the out of pocket cost for each lot.
But the sale of the RSU stock DOESN'T create compensation - that all happened at the vesting 10 years ago - so it's easier and faster to enter those stock sales directly, providing the correct basis.
