You're on a 1x7 bike - so there's not a lot of range available unless you're happy to take enormous jumps between gears.
You need to figure out if 28:12 is a high-enough gear for your normal riding, and if 28:28 is low enough for the grades you want to climb.
Best way to do this is ride comfortably in a still wind, then count your crank RPM over 60 seconds. This gives you an approximate RPM value to aim for.
https://www.bikecalc.com/cadence_at_speed tells me you'll be doing 102 RPM to ride at 30 km/h in 28:12 and 85RPM at 25 km/h. That's quite high for most people, but you'll have to decide if its right for you.
https://www.sheldonbrown.com/gear-calc.html can also be useful once you figure out what your ideal crank RPMs is on the flat.
Your bike probably has a 7-speed Shimano A050 rear derailleur, which are rated for a high of 11T and a low of 28T. It should be capable of 30T easily, and probably 32T with some luck, but 34 and above will be pushing the limits I think.
Bolting another chainring on your existing bike is relatively easy, though chainrings themselves tend to be relatively pricy. You will need to match the Bolt Hole Circle of your existing chainring.
Another option is to swap out the cranks, crankset, spider, and chainring for a double, and fit a left-hand shifter, front derailleur, and inner/outer cable. The costs will not be small, and your frame probably doesn't have the cable stops for a second gear wire.
Personally I'd keep eyes open for another used bike which has better gearing for climbs. Nothing wrong with owning two or more bikes if you have the room.