There's a holder for card scrapers by veritas:
https://www.classichandtools.com/veritas-scraper-holder/p1385?srsltid=AfmBOooyJHFBhm6ew0X-G6AhsjCkmE3ePWinHbprqc9QGmznupHW88WI
You might be able to duplicate this design, it'll basically hold the card in the slightly bent position so you do not have to do this with your thumbs.
Stanley #80 is a great tool, often use it to quickly remove stickers and labels from lumber (although that does gunk up the mouth of the "plane"), would definitely recommend getting one, not just for your floor. I believe there's also a vintage Stanley with a handle, specifically for floors, would definitely never buy that, but the #80 has many uses.
But in the Netherlands, and I think in Finland also, tools aren't as easy and cheap to find as in the UK and USA (I often buy on ebay.co.uk, but since brexit that has become quite expensive). Still though, I think I found my #80 for 25-30 euros in NL (person did not know what it was).