Using a phone for Google Maps navigation is one of the things that will drain a battery quickly. So if it is behaving normally otherwise I wouldn't automatically worry. (Cameras also draw a lot of power, as it happens).

The real sign that you have a problem is not draining rapidly when doing such things but shutting down prematurely, e.g. the battery meter says 40% remaining and the next minute it's powered down. My Pixel 2 signed its death warrant last year when it shut down while using it for GPS navigation when the battery meter said 70% - I could never trust it after that.

 

Also keep in mind that if you are in areas that have weak gps and cellular, your battery will drain as well.

 

Just going by your description,it doesn't sound like your battery issue is due to the limited number of apps you actively use but more a matter of all the typical apps and processes that run in the background. Your email app, text messaging app, weather widget, and more are all updating themselves with periodic data queries, plus a lot of basic Android functions can all add up to taxing your battery more than it needs to be. Do you have Battery Saver enabled? Data Saver enabled? If you look in your Settings >> Battery menu does it show any app/service using more power that looks to be out of sorts? There should also be a Battery Optimizer option in the Settings menu, have you gone through it and adjusted things for apps you don't use? If you have Developer Options enabled, there's a Running services option that might reveal more than just the Battery menu, and it includes a second tab for Cached services that's worth checking. If you do see some app/service using a lot more battery power, try using Force stop on that app and perhaps Clear cache. And of course don't skip just rebooting your phone if you haven't done so in a while. That's always a given.
https://www.gsmarena.com/google_pixel_3a-9408.php
But a Pixel 3a is a 2019 phone so while the battery should still be in pretty good condition it's not surprising if there is at least some noticeable ability to hold a charge. All smartphone batteries age, no matter if they're used a lot and treated badly or just sitting idle most of the time. It's just a matter of degree as to how quickly they degrade.

Regarding that phone service center, hopefully they gave you a big discount or credit voucher. But smartphones are becoming increasing difficult to fix when there's a serious hardware problem. Making phones more modular and user accessible would increase costs and require phones to be larger and heavier. Manufacturers focus on maximizing profits however, so now mechanical fasteners (screws) are few while glue and adhesives are the norm. Add in all the dust and moisture seals, and tearing down and reassembling a phone is a lot trickier than it used to be. The iFixit site has a detailed, step-by-step guide on replacing the battery for your model if you're curious or are thinking of doing it yourself.
https://www.ifixit.com/Guide/Google+Pixel+3a+Battery+Replacement/124322
If you do want to do this, get all the necessary parts and tools before you start, and don't skimp. And be patient and don't worry about how long it takes, do it at your own pace. There's going to a lot of parts and pieces so if you have another phone take a lot of pictures so you have a visual record of what goes where. The goal is to have a fully functional phone after you put it back together, with no parts accidentally left out.

 

That was a different phone, so you need to be cautious when making comparisons: different phones have different screen sizes, different battery sizes, and may be more or less efficient in how they use power. And there are other factors: if you are in an area of weak cellular coverage, or changing cells often, your battery will drop faster. If you have automatic screen brightness on then when using it for navigation your battery will drop faster if the day is brighter (the screen is generally the biggest power user on a phone, so this is not a negligible factor). So it's hard to say whether there is anything unexpected here (I also don't know how old the battery in the phone you were using in 2017 was).