I mean, many brokerages will extend margin for free during ACH transfer (or any settlement event for that matter). So using a margin account and buying the ETF will save you one basis point in fees every year. If you have a 30 year horizon it becomes ~20-30 basis points depending on contribution+return schedule. 20-30k on 10mm is not super much but not negligible either since it takes no effort.
TDAmeritrade will extend no interest margin for transfers iirc. I haven’t looked at robinhood in a while, but I believe they will extend instant deposit beyond $1k. I remember I had maybe 70k with them a year ago and my instant deposit was 50k. It still has to be no greater than max marginable value (so you can’t do it on a fresh account).
