I’m looking at these zigbee modules to go in my ceiling behind the light so that I can still use standard light switches and dimmers. I still want to be able to control the light even if the light is turned off by someone at the light switch.
Are these the best and cheapest modules to use for this? Are they easy to install?
SM309-S
That category of module intercepts the wall power switch.
In other words, the relay within that module does what the wall switch does. That 'frees' the wall switch to do something else... which is connect to the module too. You can then command the relay via Zigbee or the wall switch.
I use Zwave for the two modules I have, but they are the same physical size. They are relatively easy to install, I've found the hardest part is getting all that stuff into a wall box. Wago's sure help. 
Do you lack a neutral in the switch boxes to use smart in-wall dimmers, instead of a relay module? I do have two Zigbee modules in the ceiling for two circuits that did not have neutral in the switch box to use a Zwave dimmer switch, so I use the neutral in the fixture with a Zigbee relay instead. Every other light switch in my house has a Zwave in-wall dimmer switch.
The relay you posted does not need a dedicated neutral at all, actually, so I assume that is the reason? If you don't have neutral in the switch box, then it is in the fixture, so you still do not need a non-neutral switch since you would have neutral in the fixture box. Those non-neutral switches can have their own issues, they always put a trickle of power through your light to get power to the switch itself. That can make some bulbs glow dim when off, unless you add a bypass capacitor.
That relay you mention doesn't actually work with a regular switch, you would need to buy momentary switches. My hallway relay does dimming, but I had to buy momentary switches. Pressing the switch usually turns it on and off, and holding the switch will dim or brighten until you release it. There is no dedicated on or off (it toggles) and no dedicated dim or brighten.
So, if you have neutrals in your switch boxes, I recommend you just use smart dimmer in-wall switches instead.
I have used modules like the Samotech modules. They work fine. I've also used modules from Sonoff for the same purpose. Specifically these:
Switches: