I agree with Mike. I think stored procedures are abused, and also locks you down to a database.
If I was the decision maker for your project, I would personally approach like this (of course I haven't seen your source code so take my suggestions with a grain of salt).
1. First, upgrade to PowerBuilder 2019 R3. Compared to PowerBuilder 2019 R2, this is a long-term support version and has some really nice features. Again, I think Mike is totally right you do the PB upgrade first get on supported version then mess around with architectural changes.
2. Second, I would look at removing obsolete features. Obsolete features are ticking time bombs! This is not the complete list since you are on such an old PB version, but I would definitely recommend at least dealing with this short list.
3. Third, consider enabling some new features that will modernize your app with very little to no effort, for example UI Themes, NativePDF, PowerClient, switch out icons with the new ones provided, and if you are using SQL Server switch to the new OLE DB driver.
4. And finally, if you need to make architectural change, I would look at moving PowerBuilder business logic (e.g. embedded SQL, non-visual PowerScript, DataStores, etc.) into REST APIs. The PowerScript Migrator feature of PowerBuilder CloudPro Edition will highly-automate such conversion. You can easily call REST APIs from PowerBuilder using the RESTClient object.
Basically, simply put I would approach architectural changes last.
this is off topic, but i came to that conclusion too and went 100% in the other direction. I eliminated most stored procedures in my application. There are less than a half dozen reports that require stored procedures (very complex reports that won't work otherwise.), nothing else uses a stored procedure. This allowed a huge amount of flexibility especially at runtime. My application generally gets faster with every release - and it runs both as client/server as well as powerserver web.
In my experience I have found that I want EVERYTHING under my control, thank you VERY much! I have no desire to wait for DBA approvel to start updating my SPs, or to depend on anyone else to synchronize the movement of these updated processes into production environments. While they do have a place, they should be used as sparingly as possible at all times. Use the flexibility and power of PowerBuilder whenever possible.
Just.... no.