Best way to archive personal data at home
There exists many time-proven solutions for data archival - so there's lots of best practice advice around.
You indicate that you have no existing local network, you have a slow internet connection using a phone hot-spot and you have only about 500 GB of data collected over 20 years that you wish to archive.
In this case, I wouldn't have cloud services or NAS devices with old style hard drives in a RAID setup at the top of my list of technologies to use. Those come with recurring operating costs and other drawbacks.
Instead I would look to storing the data on optical disks or tape. Depending on the product used you have plenty of space, it is relatively cheap to make extra copies and it is local only technology. I.e. for example today you can optical disk recorders that store multiple terabytes of data on a single disk - however, I would advise you to start with one of the cheaper options ofcourse.
The advantage of being able to make multiple copies is that:
a) any copy will eventually deteriorate over time and needs to be replaced (this is also the case with RAID-volumes, where hard drives will eventually fail and need to be replaced)
b) you risk theft of the device, house catching on fire, water damage, etc. - so better to have multiple (encrypted) copies that you can store in other locations
For optical disc storage you can start with a simple BluRay. A good quality, archival grade disk probably lasts 20 years without you having to do anything except keeping scratches away. It is ofcourse best to keep them out of light, heat, vibrations, etc. as well.
I would say that with a budget of 300-400$ you can get a BluRay BDXL burner and a big pack of 100 GB archival discs so you can get all your data stored.
Another contender would be using the commonly available, very small sized, SD-cards. These are available at cheap in the capacities you seek, and it is easy to make multiple copies that can be stored in various locations (in the car, at a friends house, etc). They typically last longer than hard drives that have a life-span on average of about 5 years or similar.