I'd focus a bit on working out what you "need" versus what you "want" in terms of features. Once you've developed that list I'd then look at what models you like and don't like. I realise that you've covered some of this in your original post but given some of the suggestions given clash with your dislike of SMG's, which was absent from your OP, it makes recommendations somewhat more difficult as it reduces it to pure guess work.
The best we can do is put forward suggestions based on the information given but, as highlighted by
@Cromulon1994, we are a UK based forum and as such some of our recommendations may simply be non-viable for a myriad of reasons. Inevitably your biggest obstacle will fall down to availability.
As far as build quality of the TM Scar goes your experience differs wildly to mine but based on your experience I understand why you're hesitant/no longer interested in pursuing it as an option.
From what I've seen of the
airsoft reddit a lot of US based players recommend guns from the VFC Avalon line and many are vocal supporters of guns from
Umbrella Armory. I believe VFC produce, or at least used to, various iterations of the FN Scar and given you're open to having the internals worked on from the get go perhaps this would be a better option. Whether or not the VFC Scar externals are higher quality than the TM I really couldn't say.
I couldn't say how much of an impact flicking between 350fps and 400fps is but a quick change spring system might be worth considering although you'd probably be better off with something set up for 350fps that performs consistently with a high quality hop and barrel set up.
One thing you'll find is that once you start pursuing upgraded guns then your options open up significantly unless you're after specific features such as the bolt stop on many of the Tokyo Marui NGRS line, this is why assessing your needs vs wants is key. It's also worth taking into consideration your longer term plans for the hobby, if any. Some people know from the outset that they're going to pursue Milsim's in which case it's worth trying to decide if you're going to build a US, NATO or RUSFOR kit as these are some of the most common factions. If Milsim isn't something you're interested in then this is a moot point.
Your budget is high for a new gun but it's definitely a nice nest egg to work with. Hopefully some of what I've said is useful to you. Please bear in mind that if you're new to the hobby and haven't tried it I'd strongly recommend renting at a reputable local site once things clear up following the ongoing issues relating to the corona virus.
edit: To play devil's advocate on the ambidextrous fire selector, my old man is left handed and has yet to voice any complaints about the fire selector on any of my guns when it comes to using the selectors.