You are right that RoundUp being glyphosate + surfactants and other "inert ingredients" is significantly more toxic than glyphosate alone. And you are also right about the shikimic acid pathway. It is on that basis that RoundUp was initially touted as the ideal herbicide, toxic to plants and harmless to animals. But that turns out not to be totally true. It is a complex bioactive molecule and biosystems are complex, so there is more than one mode of action. And there are lots of ways animals can be affected.
For example: " its broad spectrum of herbicidal activity has led to the destruction of habitats and food sources for some birds and amphibians leading to population reductions(25). The Houston toad is an extreme case in that it is now an endangered species due to destruction of its habitat by glyphosate(26)." also " Fish and aquatic invertebrates are more sensitive to glyphosate and its formulations. Its toxicity is increased with higher water temperatures and pH. Some soil invertebrates including springtails, mites and isopods are also adversely affected by glyphosate. Of nine herbicides tested for their toxicity to soil microorganisms, glyphosate was found to be the second most toxic to a range of bacteria, fungi, actinomycetes and yeasts(27). However, while glyphosate alone has low toxicity, the formulation of glyphosate with the surfactant polyoxyethylene amine (POEA), which is widely used, is significantly more toxic." https://www.pan-uk.org/pestnews/Actives/glyphosa.htm
One reason that glyphosate can cause sub-lethal problems in animals including humans, even though we don't have the shikimic acid pathway is that many microbes do have it. So the glyphosate is disruptive to beneficial gut bacteria and others. This may be key to understanding how it causes harm in both humans and animals.
The bacteria in your body outnumber your cells by 10 to 1. For every cell in your body, you have 10 microbes of various kinds, and all of them have the shikimate pathway, so they will all respond to the presence of glyphosate!
Glyphosate causes extreme disruption of the microbeâ€™s function and lifecycle. Glyphosate appears to preferentially affect beneficial bacteria, allowing pathogens to overgrow and take over.
"Celiac disease, and, more generally, gluten intolerance, is a growing problem worldwide, but especially in North America and Europe, where an estimated 5% of the population now suffers from it. .. Here, we propose that glyphosate, the active ingredient in the herbicide, RoundupÂ®, is the most important causal factor in this epidemic. ..Celiac disease is associated with imbalances in gut bacteria that can be fully explained by the known effects of glyphosate on gut bacteria. Characteristics of celiac disease point to impairment in many cytochrome P450 enzymes.. Glyphosate is known to inhibit cytochrome P450 enzymes. Deficiencies in iron, cobalt, molybdenum, copper and other rare metals associated with celiac disease can be attributed to glyphosate's strong ability to chelate these elements. Deficiencies in tryptophan, tyrosine, methionine and selenomethionine associated with celiac disease match glyphosate's known depletion of these amino acids."
Interdiscip Toxicol. Dec 2013; 6(4): 159â€“184.
Published online Dec 2013. doi: 10.2478/intox-2013-0026
Glyphosate, pathways to modern diseases II: Celiac sprue and gluten intolerance https://www.ncbi.nlm.nih.gov/pmc/articles/PMC3945755/
also Glyphosateâ€™s Suppression of Cytochrome P450 Enzymes and Amino Acid Biosynthesis by the Gut Microbiome: Pathways to Modern Diseases Entropy 2013, 15(4), 1416-1463; doi:10.3390/e15041416 https://www.mdpi.com/1099-4300/15/4/1416