In my situation, I've set the Kobo Touch Extended "Collections" field to be "tags." Looking at Tags in Calibre, I've got 73 of those. But, I don't see sync failing to complete (even when I had Automatically Sync and WiFi on.
For online connections, the only things I'm using are my Kobo account, itself, and Overdrive.
All my Kobo-purchased books (10 of them) are edited and sideloaded from Calibre. The one oddity there was that when I went to sideload my Calibre library back to my factory-reset and re-updated (to 4.34) Forma, one of those books did actually get downloaded from Kobo (I must have forgotten to Remove it from my Kobo library). I've now removed it and all books are, currently, from sideloading.
Based on NiLuJe's comment, above, I've also now turned off the Automatically Sleep and Wake Up option in Energy Savings and Privacy > Sleepcover (just in case something is wonky with the sleepcover sensor).
And, as an update, after resetting and re-sideloading, I read yesterday for a couple of hours and the battery use looks about normal. I put the Forma to sleep last night at 75% battery and this morning, 8 hours later, it was 74%. Again, normal. I'll see what happens over the next couple of days (with my usual USB connections to Calibre).
EDIT: I've currently got 433 books on the Forma.
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