With your ROM version 20.12.28/29, you currently have installed, you have a TWRP installed which does not decrypt. If you go into this TWRP (Pwr & Vol+ buttons) you will be able to install the new ROM using a USB memory stick, through OTG (USB-OTG)
Using USB-OTG, and only upgrading a few versions (also weekly ROM -> weekly ROM), you should be able to install this over the top - ie. dirty install - without losing any data. Yes - you clear the Dalvik and cache, and also (if you can get to the root) delete the /data/system/package_cache folder.
It is always wise to back up before you update the weekly ROMs. You will need to research for yourself which method of backup you will be happiest with. I am not sure whether you can backup with TWRP without decrypting - I've never tried it. If you can, I think that this is one of the best methods of backup. And you may also want to backup your separate files, eg. photos, etc.