You have electrical outlets around your yard, but did you install and direct burial speaker wire from your house to the areas where you would like to place speakers?
Generally 14/4 direct burial works quite well. Most landscape systems would have an amplifier located inside the house or an outdoor utility room where the amp and source, such as a Sonos Port would be located. Then passive speakers, like you mentioned and shown above could be installed at the other ends of the speaker wire. These would be systems like:
This system is powered by a Sonos Amp so it might be OK but depending on the size of your yard, it would run out of steam pretty quickly. The larger systems have dedicated amplifiers and can have more satellites, 2 subs, etc., but the cost much more.
If you can't run wiring to the house, you could install something like this from Rockustics:
https://rockustics.mseaudio.com/os-440-outdoor-smart-amplifier.html
This would be installed outdoors, in a weatherproof box, near one of your power outlets. Then you would run direct burial speaker wire from this enclosure to some speakers like:
https://rockustics.mseaudio.com/xt-powerrock-g.html
The reason for choosing these speakers is they have a compression driver and an 8" woofer making them much higher efficiency compared to the other speakers and that small amp only has 40wpc to work with. These speakers are rated at 97dB so they will make use of the amps limited capability.
Hopefully that helps.