Around 3 years ago, org that I worked overestimated the ready-made capabilities of n8n or underestimated the requirements of the clients. Either way, we were stuck and vertical didn't work due to the workload. Horizontal auto scaling is in n8n's road map but not a priority.
Luckily for us, n8n was Open-source, and they had a master worker mechanism which we leveraged and implemented our workflows and it performed way better than we expected.
We did have to make a few modifications and create custom nodes by tinkering with their code base. Again all thanks to Open-source.
When pushed to a corner, we eventually find a way out. Thanks to Open-source, we found it way sooner.