Hello from Australia,
This thread has been very useful, so I thought I would share something i just discovered.
I have also had the issues with exporting data from the growatt web page, and I have resorted to installing homeassistant and letting it use the required API calls to sort me out, which works ok.
The trick of using the 'older/alternative' URL with plant.do in it works .. sometimes.. as soon as it works out you aren't 'new' it puts you back to the new site.
I found that you can generate a URL to 'share' your dashboard with others. It looks very similar to the 'owner' dashboard in most respects, BUT, has the 'History Data' enabled.
From the main dashboard -> Energy Button
Choose 'Plant Management'
On the far right column, I see 'Operating Tools' and one icon is for 'Share Plant'
On the new window, put in your email address, how long it is effective, and press 'yes' to send yourself the email.
When you get the email, open the link in a different browser, or an incognito window. It shouldn't need to be logged in.
Then on the dashboard, when you scroll down to the 'My Photovoltaic Devices' you have the button 'History Data' instead of 'Setting' that you can usually see.
From here you can export a day of data at a time to an excel file (not great, but, better then nothing.)
Hope this helps someone!
(tested on a SPH5000)