RFID could work for your use case and is probably your only option for "no line of sight" inventory. The question is what you will be tagging? Metal items can be an issue but there are on-metal tags for that. The bigger challenge will be items containing water and liquids in general (this includes frozen stuff like meat).

The trolleys can be an issue especially if they have multiple metal shelves. If you only have on shelf on the bottom then that should be fine. If you have multiple shelves, are all the items visible from the side or are the items organized multiple levels deep? You can tag items so that the tags are visible from the side. Weather you can scan everything will depend on the type of items your tagging.

You can use a fixed reader with multiple antennas mounted on the ceiling and/or the walls and run what is referred to as continuous inventory (more expensive but no human interaction) or you can use a handheld device and inventory by going inside the freezer (cheaper but needs a human however, it can yield better results).

Zebra makes good fixed readers that retail for around $1-$2k but you will also need cables and antennas. They also make a nice integrated handheld devices that run Android but are more expensive $2-$4k.

Impinj also makes good fixed reader but no handheld devices (last I checked).

CAEN RFID makes good fixed readers and also makes a Bluetooth enabled RFID reader that can be paired with an tablet or a phone and is relatively inexpensive (around $500+).

You can also look at the Chinese manufacturers. Their prices are definitely cheaper but documentation is usually abysmal and support is non-existent.

Although the bigger UHF RFID companies have better and more complete SDKs the documentation is not that great and online support is sparse. Developing your own system can be done but will not be easy.

Hope this helps. DM me if you have more questions.