I wonder why the RPL isn't more popular for this. It's an OSI-approved license that is meant to fix the "SAAS loophole":
PREAMBLE
The Reciprocal Public License (RPL) is based on the concept of reciprocity or, if you prefer, fairness.
In short, this license grew out of a desire to close loopholes in previous open source licenses, loopholes that allowed parties to acquire open source software and derive financial benefit from it without having to release their improvements or derivatives to the community which enabled them. This occurred any time an entity did not release their application to a "third party".
While there is a certain freedom in this model of licensing, it struck the authors of the RPL as being unfair to the open source community at large and to the original authors of the works in particular. After all, bug fixes, extensions, and meaningful and valuable derivatives were not consistently finding their way back into the community where they could fuel further, and faster, growth and expansion of the overall open source software base.
While you should clearly read and understand the entire license, the essence of the RPL is found in two definitions: "Deploy" and "Required Components".
Regarding deployment, under the RPL your changes, bug fixes, extensions, etc. must be made available to the open source community at large when you Deploy in any form -- either internally or to an outside party. Once you start running the software you have to start sharing the software.
Further, under the RPL all components you author including schemas, scripts, source code, etc. -- regardless of whether they're compiled into a single binary or used as two halves of client/server application -- must be shared. You have to share the whole pie, not an isolated slice of it.
In addition to these goals, the RPL was authored to meet the requirements of the Open Source Definition as maintained by the Open Source Initiative (OSI).
https://opensource.org/licenses/RPL-1.5