External HD and Lightroom cloud

Just reiterating that you can't use the lightroom cloud as a backup even if you use the cloud version of Lightroom called Lightroom Desktop (yeah I know terrible naming by Adobe). For Classic this is due to the fact that Classic will not sync full raw files (a very old feature request that hasn't been honored: https://community.adobe.com/t5/lightroom-classic-ideas/lightroom-classic-should-be-able-to-sync-full-raw-files-to-the-cloud-not-just-smart-previews/idi-p/12248616 ) and with Lightroom Desktop, while you can sync raw files to the cloud, it just doesn't work right with local storage of copies on a removable disk that is not always present. It is also not a real backup (accidentally delete an image from the cloud and it will immediately also be deleted from your local storage completely!) that you can really rely on. They did introduce a recylce bin feature some time ago that makes it a bit better but it is nowehere near a real backup. It really is just cloud storage of your images and you would still need to backup your files somewhere else. So backup of your images is not a real good consideration to choose between the two.

The real question is: can you live with the much reduced feature set of Lightroom Desktop. It cannot print, it cannot use virtual copies, it cannot do hierarchical keywords, it cannot use publish services (or not very well) and is missing many other features that you might rely on. It does have the very nice feature that all your images are available on any device you connect. So if you can live without the classic features and your images being online in a cloud works well for you, go for Lightroom Desktop. Do realize that you will still need to figure out how to back up those images. If you stick with Classic, get some sort of online backup solution and store your images on external hard disks that you backup to the cloud. The images will only be available to the laptop when the disk is connected. The best solution is actually to get a NAS from synology or QNAP or something similar and put your images on it. Set that up to automatically backup to your cloud backup (they all support most cloud backup solutions). Now you can access your images throughout your own network from your laptop and you can even setup a VPN connection to your home network and access these images from the road as long as you have a good wifi connection. This is what I do with a synology NAS at home and I use a vpn connection to my home firewall to connect to it from anywhere in the world.

Get your math right if all you're considering is cost. Backblaze is $70/year or $5.83/month. The $19.99 (monthly) you state is about $240/yr  - 3-1/5 times more than BBZ.

The Adobe cloud cannot be considered as your backup for photos being stored on an external hard drive. If you use Lightroom Classic then images imported are stored on the local hard drive (internal or external) and you can create collections in Lightroom Classic  and share those collections as smart previews. But you cannot share full-sized images from Lightroom Classic. If you use Lightroom Desktop (the cloud version) then the full-sized images are imported directly to tthe cloud, and you have the option to store a backup locally, but the Adobe cloud cannot be considered as your backup.