Not at anywhere near the density you find in modern agriculture. Domesticated cattle and pigs alone are 15x the biomass of all wild mammals combined. Wild ruminants are somewhat insignificant compared to that.
Also as a point of nuance, I'd like to say that I'm sure we have actually increased biomass density. If we had not, our farms wouldn't be doing their jobs very well. I'm not trying to say no increase has occurred, simply that it is less than one might expect. Unfortunately I don't have exact numbers to back this up though. I wasn't planning on writing all this when I woke up today, and it's admittedly not my field. Here's to hoping more informed people take over!
There are 94.4 million cattle in the United States and around 4.5 million in Canada according to a quick web search.
A full grown Bison weighs 1600kg, and the average weight of a steer at slaughter is 600kg or so.
There was likely a dip between the virtual wiping out of Bison in the 19th century and the re-establishment of large herds of ruminants across the Great Plains via ranching, but in terms of biomass I think we can at least say that they are in the same ballpark.
(this puts aside the extirpation of antelope and deer species from large areas of their previous ranges. There were for instance 10 million elk in North America prior to European contact compared to 1 million today at 400-500kg fully grown).
(edit: according to statista your headline number of 1.5 billion is also off: “ The global cattle population amounted to about 989.03 million head in 2019, down from over one billion cattle in 2014.” — https://www.statista.com/statistics/263979/global-cattle-pop... )
As you stated, this doesn’t need to be the case since a lot productivity gains come down to more frequent harvesting. The natural lifespan (not average due to low survival rates in the first year) of a Bison can be up to 18-20 years, while a typical beef cow is slaughtered at 18 months. Ruffed grouse have an average lifespan of 1.5 years or so, but can make it up to 7 years or more. Chickens are typically slaughtered at 8 weeks.
As for crops, large swathes of them replaced forest which were larger pools of biomass that still cycled into the atmosphere through decay and fire.