I’ve got a lovely Trek 1.2 which I’ll be keeping as a winter bike
I’ve got the older Trek 1200 SL which I do most of my miles on, brought from a mate for a couple of hundred quid. Changed saddle, stem, bars, headset, for fit and preference. Changed wheels for OpenPro on 105. Swapped the 9 speed Tiagra shifters and mechs for 10 speed 105 (though the Tiagra arguably worked a bit better and is now on the turbo bike.) Stuck on some vector pedals for power. SKS guards and some lights. I’d not want to race on it but it’s great as a training bike.
I think it might be best to upgrade the heavy wheels and groupo on that for this summer while I save for something a bit special.
Wheels would probably make the most difference for your money.
As for something special. If you just want something to enjoy summer miles and sportives on it then fine (though I’d just use my old Trek for that.) If you want to race I’d get something more functional than special as you will most likely crash it (e.g. for 1500 quid and a dip in the parts bin you may be able to put together a Kinesis Aithien, a 105 5800 group, a LB carbon wheel build, and some decent finishing kit.)
Buying a bike with disc brakes instantly marks you out as a poor bike handler/descender (i.e. ex mountain biker) and proper roadies will avoid riding with you.
Is interesting going on club runs and seeing what people ride. On the coffee and cake runs you get a right mixed bag, lots of the nicer Sunday best plastic bikes with fancy group sets. On the race training runs you tend to get older alu/steel/ti framed bikes with very functional kit on.
