A few things:
1. It is a pain to transfer funds between retirement accounts held at different brokers. I think the last time I did it, Vanguard signed and mailed an actual check to Betterment. Maybe not a huge PITA but more complicated than you think it should be.
2. You can have more than one Roth and more than one IRA. The only real rule is you cannot contribute more than the yearly max in total across all of them.
3. Most people that do backdoor Roth are doing it because they aren’t getting any pretax benefit from their IRA contributions, so there is rarely a concern about having to retroactively pay income taxes on the funds that get rolled over. That said, even if you only contributed post-tax funds to your IRA, you will get taxed on any appreciation in the investments when they get rolled over. So if you put $5k post tax into an IRA and then roll into a Roth when it’s worth $6k, you’ll need to pay taxes on $1k.
4. Depending on who you use for the IRA/Roth, taxes can be tricky. I think just this year TurboTax finally figured out how to ask questions that catch when you’re doing a backdoor rollover, but before that I had to be very careful in how I described it or else TurboTax just assumed I was rolling over pretax dollars instead of post tax.