Don't be afraid of halves
I use Mentor, which first drew my interest because of the true-count conversion and the general similarity of the tags to the doubled halves tags.
If I ever move to another count, it will be halves. That extra weight on the 5 fits its heavier effect on your fortunes.
I wouldn't bother to double the tag values for halves. I tried learning the doubled values, and my count-down times and general accuracy seemed no better. For the .5 on the running count, I mentally affix the syllable "kay", so that 7.5 is "7k" and -3.5 is "96k" in my head. If you tend to make the glottal stop for "k" when thinking it, you should pick another syllable, in case the dealer picks up on your throat pulsating like a frog's.
On Casino Verite my halves running counts were often off by the end of the shoe. Figuring that my casino results were even less accurate, I figured I was negating the advantage of halves, so I practiced with Mentor for a couple of weeks, and my error rate was far lower.
The "downgrade" to a level 2 count may have been premature: I now think I could handle halves without a problem. But I have flash cards for the Mentor indices, and know them all now, so I'm not sure any upgrade would return enough extra earnings to justify eating the sunk cost of my Mentor training plus the additional hours to adapt to halves.
For some reason, I know my multiplication tables well past 12x12, so it wasn't hard to memorize the lines for 4, 6, and 8 up to 39, so as to do true-count conversions for 5, 3.5, and 2.5 decks remaining, respectively. For example, with 2.5 decks left and a true count of 24, I immediately hear "8x24 = 192" in my head, and the true count of 19.2 pops up.
This sounds like fastidious overkill, but it's really fast at the tables. Multiplying by 2/3 and 4/3 is harder, but they can also be memorized. I haven't made an effort to do this - it seems to be happening automatically. After a few hundred times, things like "17 -> 11" pop into my head without effort when there are 3 decks left.
Mentor is friendly to cancellation, because there are the same number of +2 cards as -2 cards, and the same number of +1 cards as -1 cards. I don't think that it's as easy to count a whole table as with Zen, because you can ignore 9's in Zen, but the cards seem to arrange themselves more easily in cancelling pairs than in most counts. Could be my imagination, though.
The true-count conversion in Mentor is blazing fast for me, and keeps getting faster. I'm good at dividing fractions, so conventional true-count conversion doesn't bother me, but two-deck true-count conversion by multiplication is a paradise I may never leave.
Give halves a chance if you want the best betting correlation you can get in shoes. I wouldn't be put off by initial difficulties unless you're really struggling to get below 30 seconds to count a deck. There's something pure about halves, and the single 1.5-valued card doesn't make it significantly harder than a 2-level count.