How can I get more internal space for apps?
On a device with such limited resources, your best bet by far is to eliminate as many apps as possible.
Any useless bloatware that was preinstalled might be able to be disabled.  This will prevent them from growing.
Any apps that connect you to sites that are accessible with a browser- use the browser and ditch the apps.
A great, small, very fast browser for such a mission is Lightning.
Lightning (Lightweight web browser) - https://f-droid.org/app/acr.browser.lightning