Sorry, you won't go very far with 16 GB of internal storage.
Just consider that at leat 1/3 of them are used for the system partition, where Android installs its updates and the "system" part of the apps. 16 / 3 = 10.333, so you have about 10 GB left for "Internal storage".
You can limit the number of the apps you have in your device and/or installa them in the external SD (if your vendor allows thet), but the main space saving is on those 10 GBs, the ones used for system data is at most intouched.
In the past I tried different solutions, included "Adoptable storage" (using external SD as if it was part of the internal storage space)... it was a true nightmare.
IMHO, it's time for you to look for a new phone; go for one with at least 128 GB (or better, 256) of internal storage, 4 or 6 Gb RAM, which nowadays is the minimum required for thee newer Android versions to work decently; if it also has a slot for an external MicroSD, then you can keep yours with all you OsmANd traces and favourites.
HTH,
Max