I got the exact same incubator myself with the same thoughts but it just wasn't worth it. I ended up getting a Janoel 10, second hand on ebay, for about twice the cost of the mini incubator which has proven quite good.
To have any kind of success with your current incubator you will have to get a second thermometer and calibrate it. A water submersible food thermometer is good and you can get one very cheap online, but it has to be a small one that will fit in your incubator.
Once you know the thermometer is accurate by testing it in boiling water or ice and salt, leave it in your empty incubator for a couple of hours and check the reading against the bator. If the bator says 38 and the second thermometer says 36.5, you know that your bator is down by 1.5 degrees and can adjust accordingly (by setting it to 39.5).
I would also recommend getting a hygrometer. Where are you located? I'm in the UK where relative humidity is usually fairly high. I therefore usually do not add any water for the first 18 days then fill both chambers in the bator for lockdown.
Finally, ebay eggs can be a pain. Ive learned to only buy from reputable sellers with good feedback - and I request them to post all eggs round end up toward the postage label on the box. I candle on arrival and dispose of any with loose air sacs or cracks (rare). Then I leave the remaining eggs to settle for 1-2 days before incubationg. This way I have quite good results.