It is common to have 'internal error 500' or 'timeout 503' errors when launching operations such as CSV import, and regeneration of thumbnails, whether on shared hosting, VPS or dedicated server, the PHP process need to have limits...

From your cPanel, you have access to php.ini settings through 'Select PHP version' / 'Switch to PHP options', to make adjustments if needed.

Catalog Import CSV list (heaviest among imports)

Go in Advanced settings/ CSV import of your PrestaShop admin, you must count about 1750 products provide import CSV, for the rest you will have to partition your catalog by batch of 1750, and start again which will add the rest of the catalog. Take between 5-10 minutes for each import.

Be careful to click : 'No' for the regeneration of thumbnails, otherwise the import will be cut quickly, causing over-charging !

When importing, your admin prestashop connection will cause nginx mistake, it's OK and normal, in fact, the process continues in the background, and to know when the operation is completed, you should look in your cpanel the left column if your processor is used a lot, or 75-100%, and regularly refreshing ... once finished you will see your processor back to almost 0.

Massive update of prices, quantity, etc .... by CSV import

By the same function 'CSV import', this time you can count on the possibility of updating possibly up to 10,000 products, why ?
When importing, you have to ignore all columns where you have nothing to update, keep ID, name, price etc .... what will necessarily reduce the overall workload of the process....

Be careful to click : 'No' for the regeneration of thumbnails, otherwise the import will be cut quickly, causing over-charging !

Regeneration of thumbnails, see Preferences/pictures/images

This process is very heavy for processor, the only method is to proceed by type, eg 'Products' and choose 'All'.
Click 'No' to 'Delete Pictures', otherwise it makes it impossible to work, because it starts all over again each time.

Once launched, your prestashop admin connection will cause nginx error, it does not matter and normal, the process continues in the background, in fact, and to know when the operation is completed, you must look into your cpanel the left column if your processor is used a lot, or 75-100%, and regularly refreshing ... once finished you will see your processor back to almost 0.

Then again for the rest of the pictures to regenerate, please do exactly the same type of images of our example.
Click 'No' to 'Delete Pictures', otherwise it makes it impossible to work, because it starts all over again each time.

Once completed, you should see this time no error in admin pretashop saying all has been done normally.

For other types to regenerate, do all the same.
Click 'No' to 'Delete Pictures', otherwise it makes it impossible to work, because it starts all over again each time.