Here is how I manage mission building objects. It evolved while I was doing some building work on a project.
1. Open a new mission and name it something like "Object template". The map you pick is a minor detail but a simple map with a lot of flat area helps. I will mention the only map drawback later.
2. On a flat area of the map, open the object viewer and starting wherever you desire, start laying out objects and grouping them by whatever category you will best remember them. For example, I have all the churches grouped together, English building, french buildings, pacific style buildings, factory stuff, wharves and docks including all the raised items to fit on top of docks. Roads and railroads with all the vehicles grouped in one area and trains and trains stuff. It sounds tedious but it can be done quicker is you take one short cut. For the long lists of items like personnel, ordnance and stuff that have many small variations, you can place just a few and when you need such an item, just select that item and your object viewer will 'jump' to that area of the object list.
3. Once assembled, save the mission.
Now when you have opened and saved a new mission you are starting to built, exit that new mission open the object template, select all the objects, copy them, re-open the new mission and paste the template of objects into a neutral area. Now when you need oil tanks, go to that collection in the template, select the tank you desire, move to the area you need it placed, click to place that object. Need a German soldier? Locate where a few soldiers group, select one, open the object viewer and your list has jumped to that area of the object list and you can now scroll till you find the one you specifically need.
Is it a perfect solution? It works for me. The only drawback is there are only so many objects that a video card will support and if you start placing objects but they have no textures, you have hit that point. Fortunately, since you will delete the object collection at the end, it is more a warning and you will lessen the object count when you delete the collection.
The only map selection consideration is if the map does not have an object in it's static.ini, it will not place on the map. This is a good way to detect such limitations on a map but it means if you build your template on a winter map or a pacific ocean only map and your mission is summer or Europe, some limitation of objects will exists that will frustrate mission building.
I hope my description of this method is not too convoluted to understand. It is not perfect but I found it over time (as I got use to my layout of objects) it became quicker than scrolling the viewer especially since it allowed me to 'grab, copy, paste' many similar objects I had grouped together) and arrange them (such as building a factory) rather than one object at a time.