Hey @Peter Hamilton, congrats on your success thus far.
As with anyone posting publicly that they have a good amount of cash to invest, I wish you luck sorting through multitudes of DMs for the next year :
I'd recommend you speak with a real estate / private equity friendly financial advisor first. They can help you to see things that you may not have considered previously and recommend how much equity to keep in the primary residence.
I've seen investors who prefer taking out a new 30-yr mortgage vs a HELOC or Cash-out refi. Why? It mitigates their risk and prevents the bank from changing their loan from something comfortable like a 5 yr I/O to "principal due now or we take your property" like we saw in 2008/09.
Regarding "jumping into multifamily in order to get better returns", what many investors have found is that they can get greater returns with less risk the larger multifamily deal(s) they can buy and spread their chips across. As I'm sure you're already aware, there will be much fewer head aches with nicer, newer properties vs run-down deals with more deferred maintenance.
With the above in mind, 2000's or newer, 150+ units in desirable locations are the way to go. If your 1M can't get you there yet, raise the money or partner with somebody who can.
Another point to think about is that if you can 2-3x your 1M in the next 5 yrs, that could enable you to buy a 10MM property by yourself. It just depends on how hands-on and active with the day to day you'd like to ultimately be.
good luck!