I only use SanDisk cards in my X70. If you are using SanDisk successfully, which card are you using. I want to shoot in 4K @ 100mbps.

I have a "genuine" Sandisk Extreme Pro (95MB/s) and several Sony SF-G1UX2 cards.
The Sandisk formats forever in my Sony camcorders but not in my Sony photo cameras.
The Sony card is much faster when formatting in the X70 - I would go for the Sony card again instead the Sandisk! It works fine with 100Mbps on both a AX53 and the X70(UHDv3)!

I ordered several of the Sandisk Extreme Pro (95MB/s) cards. They seem to work fine. I have not noticed long format times BUT I have not fully filled a card. Out of curiosity, I will set up a camera and just let it record until full.

Like this card? SanDisk 64GB Extreme PRO UHS-I SDXC Memory Card (V30)
https://www.bhphotovideo.com/c/product/1276278-REG/sandisk_sdsdxxg_064g_ancin_extremepro_sdhc_64gb.html/pageID/accessory

I'm using Transend u3 128gb works for me.