Hi Bernie. Thank you for sharing your method. I have a real nice router/table setup with some real nice bits that Dad bought before he passed. I have considered your method and will probably do that on the shorter length boards. However cutting a groove in an 84"+ long board the table saw would make it a bit I feel like. Building caskets there will be a lot of long 84"+ dado cuts. I am pretty sure at some point long the way I will find a good combination of using the saw vs. router.
I have used all of them at various times. I have the "luxury" of a table saw permanently setup with an 8" dado set. I never needed the full depth of the 8" set, mostly only 1/2" to 3/8" deep, so a 6" set would do almost everything I ever need. I use the Oshlun brand in both 6" and 8" sizes with nice results. I also have a Freud set that I have yet to use. I can see in the casket making "business" you will be making a zillion dados unlike me, so if it's at all possible give yourself a break a get a 2nd table saw for dados! Many woodworkers find a used Craftsman 10" saw and dedicate it to making rabbetts and dados! Rabbets are another great woodworking joint and should not be overlooked.
Running 84" dados or rabbetts with a router would NOT be my choice. I have made 16" long dados across shelving using my router because running a long shelf board across the table saw is unweildy and leads to inaccurate dados. I have also used a RAS a great deal for 16" long dados and is my preferred method for those! I have the "luxury" of having a 2nd RAS setup for shorter dados as well!
A router table, lift and 3 HP router will cost you around $1000 and is a wonderful addition to any shop. I have 2 of those, but unless I'm doing a lot of "edge profiling".... the router tables don't get used much ...... and I don't.
Not a waste of money, just a bit of overkill., a tool "addict's" downfall.
You can get the Oshlun 6" or 8" sets on Amazon. The 6" is under $70.00, and the 8" is about $92.00. Other brands vary in price to up over $200.00. One advantage of the 6" set is the "run down time" until the blades stop spinning and it is safe to work around them, another mostly overlooked aspect of the smaller size. I'd get the 6" and move up if it becomes necessary. In running a business keep all your receipts and get a tax number with will give you access to some wholesale pricing. The receipts will be used a tax time to amortize your equipment costs over 5 or 10 years. Also keep all your yearly utility bills ..... heating, cooling and lighting doesn't grow on trees! A good income tax person would be my recommendation!