If you are earning $12k in interest then you are saying you have...

If you are earning $12k in interest then you are saying you have an average of around $270k permanently sitting in your cash account, not in shares or other higher interest assets? If you have $270k in cash on average then you must have 3 to 4 times that in equities?

What surprises me is with $270k in free cash, and assume a lot more in equities, that you are saying your average trade is < $25k in size, as that is the value to get $5/trade as the brokerage is $5 minimum then 0.02%, so the cut-off is $25k before the 0.02% kicks in, so once your trade hits $47,500 your brokerage is more expansive than Selfwealth.

For me, if I had $270k in free cash, then I would be moving the cash to:
1) Offset account for home loan, likely to be in the range 6% to 7.5%.
2) High interest accounts which can be got up to 5.5%
3) Other short term interest bearing assets such as hybrid securities with the associated higher risk they come with.

So, to answer your final question, I move my cash around as required so Selfwealth is taking $0 off me every year, bar the interest they get while the cash sits in the account for T+2 settlement on buys, which is minor, and I don't have to pay $45/month for that privilege.

Now, I am not a medium trader like you, I do 1-2 trades on average a month most of the time, so spending the time to move cash is easy and well worth it. If you are doing 10/trades a month then you are averaging 1 trade every 2 days, so not a heavy trader, but it still might become a little more painful moving cash each time, but it is still likely to be of benefit to you depending on your external circumstances (i.e. offset account on home loan like me).

But each platform has a target audience, day-traders or heavy traders Selfwealth may not work out the best value these days, but for light traders, the buy-and-hold brigade, platforms like marketech are going to cost them a lot more. Could I get better value, even with my trading pattern, with another broker, possibly; is it worth the effort to transfer my account for maybe a $2-3/trade saving, no, but it was well worth it when I originally moved from commsec ages ago and saved myself $20+/trade.