Tropica Inspiration page can easily be set to display Aquaria < 50l
you'll see some JSM & other tanks
I'm not sure I agree that smaller tanks are inherently less stable than larger tanks - less forgiving of mistakes perhaps in some sense (ie don't over stock or add livestock too soon, not much water volume to dilute ammonia or nitrites etc) but also easy & cheap to scape & change
I've the 2nd generation Mini Flex LED & have never found it too "powerful" - I always add fast growing stem plants upon startup, also plant heavily from the first, it's fine with/without CO2 BUT I have soft tap water (with notable dissolved CO2), I fertilize minimally after using Tropica's Aquarium Soil Powder as substrate
Algae becomes a nuisance after phenomenal neglect (months without water change other than top ups)
Rather than just tank size, consider what fish etc you'd like to keep in this second tank, also some dimensions are much easier to scape than others
The Dennerle Scapers tanks are popular but I dislike rounded edges as they contribute significant distortion (not a bowfront fan for same reason)
When leaving a small tank like this, feed well for a couple weeks prior, changing water more frequently, then give a good clean 1-2 days before, then leave it - do not allow anyone inexperienced to feed fish in this small tank
(I found that even setting out premeasured food was no good, as helpful friend decided I must've made a mistake & fed MUCH more ... disaster)
I'm a great fan of Tropica's Nano CO2 kits, they work very well on smaller tanks & last a reasonable amount of time
BUT you ether need to shut off CO2 nightly or set lower bubble rate (while away or as usual method)
Evaporation is more of an issue for smaller tanks, so make sure to have a glass cover to use while away
(OK full disclosure I've forgotten & tank lost more than half water volume - all survived just fine - though I'm sure it did no favour for the heater longevity)
Heater is important in these smaller tanks, use the least wattage you can find - 25watt Eheim etc will still allow adjustable temp dial, many smaller heaters have preset temp with no adjustment possible & are designed to heat X degrees above ambient (I've seen a surprising # of Aqueon heaters smoke out - even in larger tanks - so that's put me off the all-plastic aquarium heaters, this was also an issue with (expensive) Cobalt heaters)