Same issue here, --split-tags will either export to .gif or isolate frames which means you mean yet another command to concatenate the GIF frames or isolated frame files into a spritesheet.
–split-tags --sheet output_sheet.png (without customising --sheet-type) will export all tags as spritesheets one by one, one row per tag/animation. This is great in some spritesheet import workflows but in this particular case I need one spritesheet per animation.
(and --save-as outputA.png combined with --sheet outputB.png will just do the usual export individual frames to outputA[N].png before exporting sheet, but it’s a separate operation and doesn’t affect the spritesheet output)
Please vote for Additional export options / Multiple file save if you’re interested in batch export in GUI (but we’d also need the equivalent command-line for spritesheets anyway).