6 months stay without visa

My two daughters, both UK residents live in Suffolk. I am retired and plan on flying over in Septmber for a 6 month stay. I have read the UK.gov site numerous times and am fairly certain I can stay for 6 months without any visa. I might apply for a family visa at some point which I understand would allow me to make return trips over a period of time, 1,2, 5 years, etc. any comments. Cheers.

It's a visa, just one you don't apply for before you arrive.

As long as you can show that you have enough money to support yourself and somewhere to return to you should be fine.

If you want peace of mind, you could apply for a visitor visa in advance of travel.  That way, it would be extremely unlikely you would be turned away at the border.  But it's usually not necessary for US visitors to do this.

As a general rule of thumb, the longer you wish to stay in the UK, the more ties to the US you must demonstrate.  Someone proposing to visit 2 weeks will likely encounter less questions at the border than someone wishing to stay 5 months.  If the IO is satisfied you aren't an overstay risk and will leave the UK at the end of your visit, you'll get a visa at the border.  It can be up to 6 months (and usually is for most people), but is at the discretion of the IO.  You may be asked for evidence of finances, UK accomodation, travel/medical insurance, plans while visiting, and your ties to the US.  So long as you can demonstrate such things, you'll likely be fine.

I might apply for a family visa at some point which I understand would allow me to make return trips over a period of time, 1,2, 5 years, etc. any comments. Cheers.

Don't forget that once you have built up a good immigration history and can have a 5, 10 year visitor visa, that the same rules still apply i.e. no more than 6 months in a year, no free NHS ect.