well i would go to a time before industrialization and conquer the planet, why just stop the stargate from reaching the usa, why not preventing them from discovering it or simply going back after the bury the gate so you can take the earth and no civilization to worry about.
Actually 1939 is a good year for Baal but not for the way he did it.
Consider Baal knows that in 70 years Earth if they continue on the original timeline will defeat the Goa'uld, he can take advantage of that potential to conquer the System Lords. The only one that would be trouble would be Anubis.
Step #1 a/ Send emissary back in time to warn Oma Desala about Anubis on Keb before Anubis deceives her. That way Anubis does not get the advantages of having been ascended then partially descended.
Step #1 b/ Travel back to 1939, to the day the Achilles reaches port. Take a handful of Jaffa followers well briefed on Earth and key equipment. Take all the files acquired by Baal from The Trust. Break out of the Achilles and establish a base of operations.
Step #2 1939-1940 Seize Seths compound for the technology base he has. Kill Seth.
Step #3 1940 Found The Trust immediately. Take the leaders of the Axis and Allies plus the U.S. into The Trust. No Pearl Harbor WWII never becomes fully engaged as peace is achieved by 1941.
Step #4 Found the United Nations (likely about 1943) as a world government with Baal as the Secretary General
Step #5 ~1950 Free Osiris and Hathor as your minions. Use Hathors abilities as Queen to create Jaffa and drones in the Anubis style.
Step #6 ~1965 found the SGC using the information from the original Trust to choose worlds with naquadah, trinium and other materials not available on Earth to exploit that the Goa'uld will not be visiting in the next few decades. Take Jack O'Neill as host. Collect the timeship. (If Jack isn't born there will still be those with the Ancients gene to choose from).
Step #7 Use these assets to build a fleet in Sol System.
Step #8 As cloaked ships with Asgard beaming technology becomes available use them to steal gates from unused worlds and build a gate bridge to the Magellanic Clouds and create a 2nd gate network there that is not connected to the Milkyway network except at one or two secured locations (Earth and the Alpha site?). Colonize those worlds from Earth and work at industrializing them under "Trust" control.
Step #9 Build Gate Bridge to the Pegasus galaxy and take control of Atlantis. Use Goa'uld tech enhanced naquadah generators to power Atlantis. Stay on Atlantis, do not contact anyone outside Atlantis in the Pegasus galaxy. One exception might be the 1 Wraith on the other planet in the Atlantea system who might be taken as a host just to see if the goa'uld could control them. If they can be hosts work to genetically engineer a Michael style hybrid that has as many of the Wraith's advantages while also having the Ancient genes. Result a superiour host. Don't share the advanced host keep it for yourself. Alternately use it to create enhanced drones.
Step # 10 Use the Ancients technology to enhance the fleet. If Atlantis can be made to fly bring it to Earth. If the stardrive is reliable enough first travel around Pegasus and steal as many gates as possible. Large cloaked intergalactic freighters could be used to steal even more gates. Some of these gates can be used to enhance the 2nd network in the Magellanic clouds and possibly other dwarf galaxies bound to the Milky Way.
Step # 11 Pegasus gates can be used to create an "attack network" in the Milkway. When ready to attack use the gates of the 3rd network to simultaneously hit the top System Lords with the Symbiote poison. Consider hitting the most dangerous worlds with naquadah enhanced bombs instead. Cloaked ships could be in place to grab the stargates from the attacked worlds.
Of course if Baal did this then SG1 would not have used an appropriate GDO signal and would have impacted the sealed iris.