timeout expired. The timeout period elapsed...or the server not responding. The timeout in the config is an hour, why does it fall after a minute?

Question

.NET

Data is inserted into the temporary tables in the database, the process parses the data, inserts it into the database, removes duplicates and moves it from the temporary tables to the main ones within the same database, it takes about 30 minutes, as a result, approximately 10,000,000 rows are obtained in half an hour in the final tables .
The problem is in mssql server, during processing and after the whole process, it starts to consume resources (disk, memory and a little CPU (eye estimates)), presumably starts creating statistics in temporary tables, attempts to disable this behavior did not help (statistics still appear ), plus writes a huge log.
Sometimes it generally crashes with a timeout after a minute or two, although the config specifies 3600 seconds for the connection string and command execution.
Tell me how you can configure the server or optimize something? Willing to listen to any options. SqlBulkCopy is used to insert data.