My general advice is not so much about the software as controlling your corporate appetite for customization. There is just an incredible cost to bespoke customization, while the use cases of it are often things like executives' vanity at getting live dashboards or something else. You can have an in-house team build some forms atop a database, and you can even make that work, but only if you rigidly fight feature creep. If you can't fight the feature creep, might as well go with one of the big solutions so that features are reflected in bottom line costs and have to explicitly be paid for out of your business budget, rather than implemented by making requests of your in-house dev team.

Fix the business processes first or you risk codifying bad business processes in the platform.

This, also if you want a good piece of software to fit your needs the current process must be well documented and reviewed many times.

I am currently working on a large project, which unfortunately needs to communicate with DB2 and interact with AS400 (old ERP system). At this point nobody knows how things work and how the business operates exactly. Managers hold pieces of information but it is not documented at all. I have hit many road blocks because one thing was said, which turned out not to be true after a bit of investigation.

I could develop this software in 3 months, but it is taking 1+ years due to lack of documentation, developers not being on the same page, and nobody understanding the current systems.

Yes. Hire an architect with a proven track record and significant experience and give them the responsibility. Reading a few medium articles or an O’Reily book won’t be a substitute.