I don't have experience transferring taxable brokerage accounts, so requesting your feedback please.
I'm transferring a taxable account from Vanguard to M1 Finance. It currently holds VTSAX (Vanguard total stock index fund). On the phone, M1 told me that since they don't have mutual funds, they liquidate my VTSAX and put the cash in the M1 account. And add a $250 transfer bonus.
I'd like to ask, will doing this be very bad or disadvantages from a tax perspective? I want to make sure that there won't be a big surprise in the future that will make me regret this.
I've never sold my taxable holdings before (nor transferred accounts), so I'm ignorant of any negative tax implications of that. I'll be grateful for any guidance from you all. Thank you

[Editing to add:]
I am also transferring my Roth IRA (which also holds VTSAX). Should I do anything here, like convert it first to ETF (VTI) and then transfer?
[Edit 2:]
In M1 I plan on buying UPRO, TMF, EDV. (It's a small % of my portfolio and I know it's risky. So hoping not to have this thread about that, but rather just the mechanics of transfer

From the comments below, I'm getting the feeling that it's not a good idea to transfer the taxable account. So I am thinking about only transferring Roth IRA to keep it simple.