If someone day-trades crypto (like Bitcoin) within Coinbase limits, would they be considered a “virtual currency business” requiring a money transmitter license?

Not if you do the following:

1. Trade for yourself only.
2. Are not executing trades on behalf of others, or on behalf of others who have given you money?
3. Your transactions is not termed as an intermediary transaction when being handled by you.

Also see: Faisal Khan's answer to Is it legal for US citizens to engage in cryptocurrency arbitrage with an international partner (i.e. buy BTC on Coinbase, 𝙳𝚎𝚛𝚎𝚌𝚝 𝙲𝚘𝚗𝚝𝚊𝚌𝚝 𝚑𝚎𝚛𝚎 »💯☎️📲 (☞ +𝟷»𝟾...𝟶..𝟺.⸗.𝟾..𝟾..𝟻.⸗.𝟶...𝟼...𝟼...𝟼 ☜ ) 𝙲𝚘𝚒𝚗𝚋𝚊𝚜𝚎 𝙾𝚏𝚏𝚒𝚌𝚒𝚊𝚕 𝙲𝚞𝚜𝚝𝚘𝚖𝚎𝚛 𝚂𝚎𝚛𝚟𝚒𝚌𝚎 𝙽𝚞𝚖𝚋𝚎𝚛 ?