If I am not mistaken, Linux Mint settings service uses xkeyboard-config.
Would you mind to add "caps:return" to the list of supported keyboard options?
Source: https://gitlab.freedesktop.org/xkeyboard-config/xkeyboard-config/-/work_items/121 [2017-04-07]
---
xkeyboard-config 2.43...
Added caps:return to make the Caps Lock key an additional Return key.
Source [2024-10-01]
---
<option>
<configItem>
<name>caps:return</name>
<description>Make Caps Lock an additional Return key</description>
</configItem>
</option>
The actual commit: 6b30f36201b40fddcf28918eb4c96b13a23b42ae
Yet, I've checked in a virtual machine online, and there's no such, apparently, indeed (sorry).
However, I believe it's possible to modify it manually in file /usr/share/X11/xkb/rules/evdev.xml, and I've tried quickly the following (after changes to the file, restart the csd-keyboard; e.g., by killing it). Would it work in your case (I would also try rebooting after)?:
Screenshot