expand and nextLink

[derkod]

Hi,
I have this URL to extract Things of a specific project and expand it to Locations and Datastreams/Observations
https://api-samenmeten.rivm.nl/v1.0/Things?$filter=contains(properties/project,%20%27HEI%27)&$expand=Locations,Datastreams/Observations

It returns 72 Things with its Location and I see Datastreams and Observations. So far so good.
The results per page is set to 200.
I expect far more than 200 Observations per Datastream.

Questions:

1. I do not see a nextLink under Observations. They are limited to 200. How can I get all Observations?
2. How can I filter on phenomenonTime. I know how to do that using only the Observations endpoint, but not using this expand-feature.
3. Is 200 result per page a good choice? The default is 20. We already changed it to 200. What are the contras for setting it to eg 20000?

Thanks in advance,
Derko