In general, end-of-life (EOL) is generally marked by a new version of iOS (or for iPad, iPadOS) requiring more system resources (CPU or RAM) than the installed hardware can provide.
Consider that all iPad Pro models have modern/powerful 64-bit CPUs, with 4GB RAM; the only exceptions to this are the original 9.7” iPad Pro (2GB RAM) and the newest top-end iPad Pro models with 1TB internal storage (6GB RAM).
All “current” models of iPad have 3GB RAM minimum. So, with the exception of the 9.7” Pro, all Pro models generally exceed - or at least match - current models for CPU performance and RAM.
Whilst we cannot speculate as to the actual EOL will occur, if linked to necessary specification, it might be reasonable to anticipate ongoing support for older models of iPad Pro until such time as the minimum hardware specifications of the then “current” models exceed that of the legacy models. iPadOS 13.x currently requires 2GB RAM as the minimum supported spec’; all devices with 2GB RAM are, perhaps, at greatest risk of EOL at time of the next major update to iOS/iPadOS.
I hope this is helpful in informing your eventual decision.