I’ve had an xNT (an X-series HF chip without the LED) installed in the same location for about 5 years now. It has slowly migrated up and in towards my wrist over the years (and rotated a couple of times).
I also use Siefly locks (specifically: Keyless Entry Fingerprint Smart Lock with Handle & Keypad: Sifely Biometric Digital Electronic Lock with Code, Electric Deadbolt Door Knob for Front, Interior, Exterior & Bedroom Doors (Black) - Amazon.com) and have found that I get more reliable reads if I intentionally activate/energize the keypad before presenting my chip. It took a little practice, but I can normally unlock on the first try around 50% of the time, and within 2 or 3 tries about 90% of the time. But maybe that behavior is different for my model than yours.
When I first got the lock I played around with one of Midwest Gadget’s field repeaters and an NFC field detector. It seems the area with the strongest NFC field was near the 3 and 6 keys on the keypad. I didn’t want to have a sticker covering the keypad so I never actually installed the repeater. In theory you could also put the repeater on the inside of the cover, but I haven’t opened the housing to see how feasible that is.