Open access resources - OER Commons resources that are graded by levels
https://www.oercommons.org/hubs/k12
The OER resources are organised in a variety of ways e.g.
https://www.oercommons.org/curated-collections
https://www.oercommons.org/hubs/open-textbooks (open textbooks)

OpenStax from Rice University has senior texts
https://openstax.org/subjects/view-all

Wellcome Trust has STEM resources
https://www.stem.org.uk/big-picture/resource-collection

Humboldt University guides
https://libguides.humboldt.edu/openedu

BCcampus Open Education (funded by the British Columbia Ministry of Advanced Education, Skills & Training
https://www2.gov.bc.ca/gov/content/governments/organizational-structure/ministries-organizations/ministries/advanced-education-skills-training

Hewlett Foundation
http://www.hewlett.org/https://open.bccampus.ca/browse-our-collection/find-open-textbooks/

Lumen Learning has quite bit but it is hard to find exactly what you want on the site itself — try a google search of lumen and the subject e.g. lumen biology produced this https://courses.lumenlearning.com/boundless-biology/
The physical geography resource book
https://courses.lumenlearning.com/suny-geophysical/

Also try https://libretexts.org/#

There is quite a bit of overlap i.e. you'll see the same resource on multiple sites.