Yes, both of those would work.

The "service with an API" approach dodges the GPL by keeping the client and server separated in separate processes or even separate servers, which is fine. (The AGPL was created as a stronger version of the GPL which prevents this dodge.)

The "proprietary plugins" approach dodges the GPL by having the end user be the one to combine the main GPL program with a proprietary plugin. See https://www.gnu.org/licenses/gpl-faq.en.html#GPLAndPlugins

That FAQ has a lot more information which you may find useful.

The sibling comment does not know what they are talking about, like most people when it comes to the GPL.