An issue to consider having your photo library backed up from your phone's internal storage to a microSD card is your backup is also in your phone. If you lose your phone, or it gets stolen, you lose your all your photos and videos. So yeah, you have a backup but only in name. Practically, it's just not a safe way.
You might want to go into the Settings of your Camera app, change the default save folder from its default internal storage, and use your microSD card instead. This will at least free up some storage space in the internal storage. But one aspect about using a microSD card as the default save media for video content is a conditional matter -- if your phone model supports recording high resolution video (i.e. 4K) you do need to make sure your microSD card is also new enough to support this. Cards come in various 'speed classes' so check you card:
https://www.sdcard.org/developers/sd-standard-overview/speed-class/
(Keep in mind this pertains to if you want to use the card as the default save media. If you're just doing the usual file transfers copying files to and from your card, it's not a vital issue.)
You could use the Google Photos app as a backup utility. It's an adequate photo manager app and you might like instead of the Gallery app, or not. The user interface is different enough.
https://play.google.com/store/apps/details?id=com.google.android.apps.photos&hl=en_US&gl=US
Don't forget this is a Google app so it's of course integrated into your online Google profile. If you're weary of Google's data mining, bypass this app. But it does include a handy backup and sync function that runs automatically in the background. Your entire photo library gets backed up into your online Google account, so you can access your photos on your phone like you usually do, and do the same using a web browser on a desktop or laptop elsewhere by logging into your account.
https://photos.google.com
Another option that gives you complete control over where you backup and sync your data is Syncthing. You can set it up to back up to a computer or to something like an online server like Digital Ocean. Unlike Google Photos, this is a completely Open Source project so there are no ties to any corporate interests and no targeted collection of user data.
https://en.wikipedia.org/wiki/Syncthing
https://play.google.com/store/apps/details?id=com.nutomic.syncthingandroid&hl=en_US&gl=US
https://syncthing.net/
Note getting this set up does require you pay attention to the initial configuration options. Google Photos is pretty simple but its Settings menu options are much more limited since a fundamental reason for its existence is to optimize things in Google's favor. To implement Syncthing,
you make decisions to optimize its functionality to serve
your needs, not some corporate data vacuum.
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