Standard visitor visa seems like it's not long enough.
I would like to come and go as I please. I would like to add I'm currently in the UK, have been here for 3.5 months.
There's no way to do what you want. As a US citizen, there is no visa you can apply for which gives you anything above your visa-free entitlement to apply for entry for 6 months at the border, as you have been doing so far. Unless you apply for a settlement visa.
I will be traveling to Germany soon for work, then coming back, then leaving the UK for a few months, but would like the option to come back after that if I want to (September 2020).
As a US citizen, you can use the ePassport gates, as I'm sure you know. These gates use an unknown function to determine whether an individual should be rejected and sent to speak to a border officer. It's likely that long stays in the UK are a factor in determining whether you'll be required to speak to an officer and justify the purpose of your visit, and your ability to support yourself.
I work remotely, will continue to do so, and am able to show any authorities that I don't plan on moving here permanently.
You are currently breaking immigration law. You are not allowed to work while visiting the UK. It doesn't matter that this work is being done remotely. Doing a little remote work while on holiday is a bit of a grey area, but if you are staying for long periods in the UK, while depending on remote work to support yourself, you are certainly breaking the law, and if found out you will be denied entry to the UK, and will possibly incur a ban as well.
I want to avoid being denied entry in September as I will have stayed here from March 10-July 17 2020, which is encroaching on the 6 month limit.
If you're planning to live long term in the UK, you should apply for a settlement visa. Since you work remotely for a foreign company, this could not be a Tier 2 work visa, unless your company has a UK division which is willing to sponsor you. You could consider the Global Talent visa if you are highly skilled and work in a qualifying field. The remaining option is a family visa on the basis of your relationship with your girlfriend. Since time spent with her while visiting the UK does not count as cohabitation for the purposes of the unmarried partner visa, you'll need to marry or enter a civil partnership with her to qualify.