Have you tried using the "Sum" function on the SP list? Test example:
1. Create an empty app and add a button (Buitton1) and a label (Label1)
2. Button1.OnSelect=ClearCollect(SPList, {name:"one",val: 10},{name:"two",val: 20},{name:"three",val: 30},{name:"four",val: 40}) This is for faking your SP list.
3. Label1.Text="Total Sum of column 'val' is " & Sum(SPList, val)
4. Click on Button
For the "filtered" columns, do you mean the columns filtered in Powerapps (Filter function) or in SharePoint? You can certeinly calculate the sum on a column that is filtered in Powerapps. For example, you can hide a column in Gallery or DataTable control and still calculate the sum by running the formula on the list (as opposed to the Gallery.AllItems for example). I don't think this will work with columns filter in SharePoint though.
