Hello!

I've got my first impreza hatchback (2019 manual trans) and want to install some baseline mods to make this thing more reliable. I bought it at 40k miles and it runs well so far, but I like driving slow cars fast and want to get some ideas on how to make sure I don't starve the engine of oil or something like that. I've never worked with AWD, boxer engines, or even double wishbone suspension before so this is all new to me.

I know the WRX and STI models have performance oil pickups/baffles and a variety of air/oil separators avaliable but I have no idea what the cross compatability is for the base models (if there is any at all).

Any suggestions on mods, brands, and general tips would be much appreciated!

1. Relax. Your car is reliable, adding a bunch of crap to it for reliability is playing into the marketing of the mod makers.
2. Open your owner's manual and get to know the maintenance schedule. Find out the recommended fluids and use those. Your trans can be touchy with fluids so only use OEM or Motul 300V gear oil. The cooling system uses Subaru's long life coolant so top offs should just be a splash of water as one should not mix coolant types/brands and swapping it sooner is a waste of time and money. The rest of the fluids there is no worry, use Dollar Store fluid to super blingy while erring to Dollar Store. Swap when the manual says to swap without getting OCD about it.
3. Get to know your car's oil consumption when you settle on whatever oil you choose. I used SuperTech from Walmart and my car does not eat much/any oil so I dump a whole 5qt jug in it and go about my day. Yes, its slightly more oil than required and have been doing so for 20 years....its not that serious. If your car goes through some oil keep up with adding some ever month or two or try a different oil to reduce consumption, either option is fine.
4. You will never need a new pan/baffle unless you legit race the car.
5. The odds of you needing a pickup tube are hyper small as the design has changed since they were invented and the failure rate is super low. Mine is OEM and 23ish years old and still kickin. For most swapping it is a lot of money and drama for no benefit.
6. An AOS is mostly for turbo models and again, drama and benefit as it adds in many leak possibilities and there's maintenance. A cheaper and less dramatical idea for non-turbo Subarus is maybe just run premium for extra protection.

no such thing as a mods to increase reliability
That's called a stock vehicle, which it sounds like you have now

even if aftermarket parts look a little bit cooler under the hood doesn't mean they are actually helping

Literally every thread on here with guys asking about how to solve their half dozen engine codes start out with a mile long mod list first.

If you don't beat that car to hell and do responsible maintenance you can easily put another 200k on it

There are parts available than what comes on your car. But will it make the car more reliable? Maybe. For example, radiators with plastic end caps like ours and Lotus and BMW and lots of others sometimes fail. Subarus tend to fail where the hoses connect, forming a crack there. But it can be many hundreds of miles before this happens. Our 2013 Crosstrek went over 200k miles without a radiator failure. When I had my Lotus, because getting to it is an 8 hour job, I replaced it during the winter when it was in the garage with a Pro Alloy all aluminum one. Subaru aluminum radiators are available for the same modification but do you need to spend $500? Maybe when you hit 200k miles, you can think about it. Ask Ron (Unabomber) as I believe his WRX has well over that mileage.

We don't have the obvious and stupid problems a lot of brands have.

Can’t really make your car “more reliable” but keeping up with maintenance will make it more reliable. 5k mile oil changes. And fix everything that breaks

Ive had no major issues with mine thus far. 2018 Impreza FB20 with a 5 speed.

Looks like your Impreza is a Direct Injection engine. Since fuel is not bathing the intake valves through the intake ports, this will eventually create build up of carbon and oil on your intake valves. I walnut-blasted the valves on my FA20 DIT (WRX) at about 35K miles. There was a significant build up of carbon/oil on the intake valves. This can lead to poor performance, reduced fuel economy and pre-ignition (pinging).
I walnut blasted my intake valves and installed Radium catch cans to reduce the oil/vapors in the EGR system. I also disconnected the EGR system which tends to bake the oil fumes on the intake valves, but you can no longer do that on a WRX without "check engine light" (CEL) issues thanks to Cobb Tuning no longer supporting the mods. The catch cans and EGR disconnect dramatically reduce oil build up on the intake valves.
If an AOS (Air Oil Separator) or Catch Cans are available for the Impreza, it would be a great addition to mitigate this issue. Also, changing your oil close at 3K miles would help as well. Some people use injector cleaner like Sea Foam intake cleaner at regular intervals, but I have heard of mixed results. Other than that, follow your factory scheduled maintenance recommendations to help extend the life of your car.
I always replace spark plugs, air/oil filters, and oil well before the scheduled intervals. Flush your brake fluid at 36 months/30K miles. I was shocked to see the corrosion in braking system after 35K miles. Use high quality fluids such as Motul products. It's worth it in the long run.
Take car of your car and it will take care of you!

secret to subarus is going fast on stock parts as much as possible. Good brake pads, good tires, enjoy it. An AOS is the main true reliability mod for these cars, but as others have said, with an n/a motor its not really necessary for you