I like this game – I played for ages on he builder before buying my 145 buy my budget was £2300 maximum so my bike is lower spec than yours will be.
With your budget I’d pick the following upgrades:
Rockshox deluxe rt3 (not the super deluxe – with what you say you are riding you won’t notice the benefit very much)
Rockshox Lyrik rt3 160
The full bash guard option (£42)
Sram GX Eagle
Sram Guide Rs or Re (depending on the lever feel you like). RS has the nicer swinglink lever with a bearing in the pivot. The RE is the R lever with no swinglink and a bushing in the pivot but with the Code DH brake caliper.
200/180mm centreline discs. May as well do this for a cheap upgrade to get stronger brakes.
Dt Swiss E1700 spline wheel option
Minion dhf 2.5wt exo 3c front tyre and minion dhr2 2.3 exo 3c rear tyre (in the summer I’d swap the back to an aggressor for faster rolling).
I’d leave the stem / bars all as the standard options.
I’d add the raceface aeffect -150mm dropper post.
Total cost £3164.
If you decided to go lower on budget I’d go slightly cheaper on the wheels probably – or drop down to Yaris for the forks.
I’ve got GX 11 speed / deluxe rt3 shock / Yari 160mm rc / Dt Swiss e1900 wheels / same tyres as above / Sram Guide R brakes (rs was out of stock and re wasn’t available at the time) and the 200/180mm discs. The only option was a reverb at the time for a dropper so I bought a brand x Ascend 125mm dropper which I love so far (150 wasn’t available at the time).
The bike is awesome – the only change I might make at some point is to add the Lyrik rt3 charger damper to my Yari’s along with a Luftkappe. That’s not to say they need them, more because I like upgrading / tinkering.