Data Load Issue using Multiple tables append and Multiple left merge queries

Here, the issue is related to the memory of your Power BI desktop

My advice would be to load a smaller dataset for the reporting purpose with a paramter, then you published your report and you modify the parameter to get the whole dataset

To achieve that, this is the steps I followed:

• In the Power Query Editor, go to the Home tab.

• Click Manage Parameters > New Parameter.

• Create a new Decimal parameter, the idea if this value = 0, Power query will load the whole dataset 
  So for a first test, set this value to 1000

Then 

• Open the Advanced Editor (Home tab > Advanced Editor).

• Find the last step of your query before the final in statement.

• Add a new step after it to apply the conditional limit.

ConditionalLimit = if Amount = 0 then
                           #"Changed Type" 
                       else
                           Table.FirstN(#"Changed Type", Amount)

in
    ConditionalLimit

Note: in my case, I called my variable Amount and the step juste before the last step was #'Change Type', in your case, you have to replace it by the name of your previous step

After that you published your report and in Power BI services, you modify the parameter to 0

Once the whole dataset is punlished, you can also use an incremental refresh to get better performance