If you find Femto too easy clearly you should be getting your teeth into a crunchier version. Of course, it's really for you to decide what this might look like, but here are a few suggestions for you to work through that should keep you out of trouble for the next year or two.
A) Perhaps you feel the numbers on the cards are too easy. Try a new set. Here's a set chosen at random: 16, 18, 24, 27, 36, 48, 65, 72 .
B) Or use a set with three-digit numbers.
C) Perhaps it's not that the numbers are too small for you, but too big. So change the numbers on your basic Femto set from 2,3,4,5,6,7,8,10 to 1/2, 1/3, 1/4, 1/5, 1/6, 1/7, 1/8, 1/10 .
D) Still too easy? Here's another set with small numbers: 1/2, 0.3, 0.4, 1/5, 1/6, 0.7, 0.8, 1/10 .
E) Perhaps it's the doubling rule (Rule 4) that's too easy. you'd better see what happens when you play with this version of Rule 4: The round is won by the higher value cards, unless the higher card is more than three times .... (Or two-and-a-half times, or ....)
I guess all these may be too easy as well, so let's move on. You may need to extend your pack to 12 cards for these versions:
F) Change Rule 3: in each round of play each player puts out TWO cards face down. Change Rule 4 as well so it's the SUM of your two cards that decides who wins. Or the difference. Or the product.
G) If you can handle all these and you still think things are too easy then I'm tempted to ask you to use a pack like this: 0.01, -1/4, 2/3, -1/10, ....
At least some of these should meet your wish to have a good game, and your noble desire to have reduced skive versions as well.
Many thanks for your interest. I hope you have some more fun coming up with your own versions that are still a lot of fun but make you sweat a little as well.