You need to get a bit more efficient at the maintenance and unloading by the sound of it! An hour to unload and blow off, wtf! 🤣 Backpack blower on site, get home, unhitch or forklift off. Let alone and hour and a half to load up and check the oil on a 14hp single cylinder petrol, don't be silly. And perhaps try keeping a few ten litre jerry cans handy, filling up at a garage is a total waste of time in my book, I have 150l of petrol and 2000l of diesel most of the time.
I take your point about pricing and what you can earn bricklaying, but I can't see many takers at £600 a day although good luck if you get that rate, I'm not knocking you, anyone can be a busy fool and it's good you value your time. At £450 a day plus any blade damage this mill will easily pay it's way (it costs about the same as an auger plus breaker for a mini digger for crying out loud). It's a relatively unique service that can easily be sold as a bolt on to a tree job when the rest of the kit is already on site. No brainer.
For me, it's not about what I can earn bricklaying (which I hate anyway 🤣), in my business it's about how many times you can duplicate yourself by putting men in the cabs and behind the controls of your machines. I then just drift about and do what I fancy (usually welding and running the yard). Works for me. But if you are a one man band then I agree it's best for you to make every day as profitable as possible. For me, there's a sweet spot and I need to keep a volume of work/recommendations coming as well.
I'm also keen to see how much I can save by doing things like halving sleepers for thinner sleeper walls (might warp too much but worth a go), milling my own oak or chestnut posts (£110 net to buy here, I resell on the quote for £160, got to be some money there) and milling basic things like 4x4 posts (costing £16 to buy these days). My local sawmill are great but everything is a month's lead time.