In US equity markets, retail get tighter spreads than institutions and lower transaction costs. Retail can trade at tighter spreads than the nbbo with ‘zero’ commission (yes, there is still payment to the broker, but this commission is missing out on an even tighter spread). Institutions trade at actual exchanges, pay fees for trades, fees for connectivity, fees for market data, etc. Institutions can have advantages with eg cheaper financing.

I think most of what you have written is basically false, at least for US equity markets.

The reason institutions like trading with retail (for equities in the US) is not that they get to charge big commissions. It’s that they don’t have to pay for the high risk of adverse selection. I.e. trades with retail tend to not be regretted because the market is not particularly likely to quickly move in the direction that favours the retail trader.

If you are retail and you have some actual edge, you could find the brokerage which gives the tightest spreads (mostly this means the one with the worst traders except some take much more pfof). But there are likely better ways to get leverage than trading on your own account, eg selling a newsletter or working for a trading firm where you have more capital.