I am using PayPal PLUS (REST/PHP SDK) and want to switch to PayPal Checkout.
Now I had a look at Payum and Omnipay. Would like to use one of them, perhaps Payum.
But, express-checkout is not the same as Checkout?
https://github.com/Payum/Payum/blob/master/docs/paypal/express-checkout/get-it-started.md
!=
https://developer.paypal.com/docs/checkout/
Express Checkout was the name of the original API PayPal created for integrating its checkout, 15+ years ago.
There are a number of newer ways to integrate PayPal Checkout now. The best UX is to use PayPal's JS SDK, which keeps your site loaded in the background and opens a small window for payer approval, with your site still open and visible (dimmed) in the background.
Here's the frontend code to use: https://developer.paypal.com/demo/checkout/#/pattern/server . Your second link goes into other details about it.
As far as the backend, any of the PayPal APIs will still work, but the most current is v2/checkout/orders so a system that uses it will be the best choice, vs. the older v1/payments REST or even older "express checkout" classic nvp APIs.
I looked at what Payum has and didn't see anything for v2/checkout/orders, only the older REST and classic nvp APIs.