All of a sudden I've spent a lot of money on my £355 bike
I'm not in a massive rush to make any decisions but I am at a bit of a cross roads as to which way to go. Replace one, replace both or replace neither and make the current ones a little better.
The top part matters, legitimate opinion (as I'm full of them) - spending the money on the gravel bike sounds like a better investment for you, it has Disc brakes already, which can only get better by going newer+hydraulic. It sounds like it fits you better and you're enjoying your time on that more, so you'll enjoy the upgrades more and feel like you get a lot more from them. The gravel bike will already have far more clearance than the rim brake road bike, so the newer trend of wider wheels and tyres won't be an issue for it. The older rim brake you can easily get into clearance issues, it's only really the last 5+ years that road bike frames came with more and more clearance. Yours being alloy it likely means you can't go wider than 19mm rims and 28mm tyres. The gravel bike should clear 32mm tyres on wider rims.
Upgrading from alloy to carbon is a big thing. It may not feel it at first, but overall riding, particularly on typical UK roads, is just much better and nicer overall with a modern carbon gravel/road frame taking so much buzz/vibration compared to an older alloy road frame.
Most of the good things you listed about your gravel bike - are reasons I enjoyed switching from an ally rim brake bike (Giant Defy 1 2015) to my carbon disc (Specialized Diverge 2016). It became my '1 bike to rule them all' for 8 years and I loved it. Still do. Yes it was an early gravel bike with road gears so fitted in well me riding it as a 'do everything' road bike, but it was immediately more comfortable than the alloy frames (on alloy wheels and before tubeless). Had tons of clearance for better mudguards and wider/bigger rims and tyres. Through the 36,000km it has ridden, I upgraded from cheap plastic guards to full metal guards. Upgraded mechanical 11s 105 to Di2 11s. Upgraded cheap alloy wheels to fancy carbon deeper road ones. For a £1650 bike I've easily spent 5k through the years upgrading and replacing things as I went. It's still fantastic to ride now and has become my commuter + winter bike. Well worth it!
The alloy Giant Defy I had, I did ride for many years (2012-2016) as my main bike, after that I rode it indoors until cracked the frame riding it on Zwift in 2021 (warranty replacement Contend still going well - has only ridden outside 3 times). When outdoors+commuting the Defy did 21,000km and I did upgrade the wheels and groupset+brakes (non-brand/FSA/Tektro to 105-Ultegra) on it during that time. For a £900 bike changing to £140 better alloy wheels (Khamsins!) made a massive difference to it, I even put £350 carbon wheels on it, much lighter but wasn't much better a riding experience on typical roads compared to the alloy. I constantly had indexing issues due to mechanical/cable wear and tear which could also be related to badly fitting mudguards on narrow wheels with tight fitting clearance. The frame was just so limited in options due to it. Changing the Tektro brakes to Shimano was a big change - as was using shimano pads. Worth doing both on yours to see if it gets much better, but long drop rim brakes are pretty limited. They just lack strength due to the length of the arm & cable pull. Fitted a Shimano R451 long drop caliper to my other halfs rear last year when changing wheels to get clearance for better guards. Worked ok but still such a fudge to get a good fit on both and no real change to power from the Claris short one she had before. But at least she's stopped moaning about getting a wet arse quite so much!
I'm a carbon frame convert, carbon wheels too can give you lots more options - especially wider and tubeless as I'm a convert for those too! But a lot of it is spending your money in the right places over time, spend it where you'll get tangiable benefit the most. Wheels. Tyres. Brakes. Groupset. If your current frame can do what you want to do, with those upgrades, there is little point changing it.
I spent quite a lot of time doing upgrades when things needed replacing, to benefit myself in future. Getting everything to Shimano cranks. Getting everything to 11s groupsets. It just meant being able to 'trickle down' components - my other half is riding a 12 year old alloy giant avail 4 (which came with Sora+Claris) with 11s Ultegra on it and at some point I'll make the call to put her on the di2 setup that is on my Zwift bike. Everything will then be electronic shifting.
I could only 'justify' my Tarmac SL7 (2024) because they where a good price & I could upgrade the parts myself to keep the price down. I felt I deserved it and the Diverge doing all my commuting was getting pretty tired (scuffs/knocks/peeling paint lacquer/stripped TA threads/broken mudguard mounts/loose BB shell). Honestly, I did expect more of a difference with the Tarmac to my heavily upgraded Diverge for the £4500 the SL7 cost (or the £6500 replacement when it got smashed). But equally it isn't really much less comfortable (most of that must come from carbon + wheels/tyres), while only being a bit quicker - 45mm deep carbon wheels compared to 30mm. I notice the weight difference more (SL7 is around 8kg and the Diverge is around 10kg) it just feels more nimble, turns quicker and I've got it setup more aggressive so I'm more aero... But my Same Diverge setup these days would be the same price so maybe the Tarmac wasn't really an upgrade, more of a side-grade...!
I'll bounce off AI for a bit and see what comes out of the mix.
