Recommendations ?

Welcome to the forum vansunet!

I do understand your interest in longevity...lol.   I have purchased 4 DS rigs since 2011...and all still work great and never fail to impress.   DS makes very high quality gaming rigs to last!    You could need a video card or drive upgrades over time, but that should be about it to get you 5-7+ yrs down the road.    

You asked about the liquid cooling...The DS HydroLux Lite CPU cooler should only require a small coolant fluid top off perhaps after a year or two of service keeping the CPU cool in the small Bolt X case.    Many years down the road it could possibly require a fluid change out for nice clean coolant, but a plastic squeeze bottle and spare liter of coolant is about all you would ever need.   The coolant is special non conductive fluid.

Your plan of going after the Star Wars Titan Xp leaves you responsible for a crucial part of your build they can’t stress test in the rig before shipping it to you.   Be advised that DS can actually order just about any specific brand or model graphics card you want, and then DS could properly finish and fully stress test your build with the card in place and your DS warranty would cover the card as well.   You might ask them about the Star Wars Titan Xp and see what you can work out.   They will place it on a PCIE extender socket as pictured so it presents upright in the case front window.   

Your build will work great. Nothing wrong with it. 2 things to consider....you are on a mini ITX motherboard, so you only have 2 RAM slots. RAM memory is fairly inexpensive right now, and any future increase in quantity means replacing the two RAM sticks with two bigger ones.    So on this build, if you think you might need or want more RAM for some reason like using the pc for CAD or graphic/ video encoding etc...then it might be best to go 32 gb now. Otherwise, just for gaming use you are fine and 16 gb RAM is all you will likely ever need!

Given your budget, I put another build together that improves on a couple of key components and costs a little more, still well under budget.   I boosted your CPU to the stronger i7-9700K 8 core processor...a better top level processor that overclocks great.   
Also, I upgraded to 16 gb Corsair Vengeance RGB RAM...better warranty and RGB colors you can control using Corsair iCue software.
Last, your Asus Strix mini ITX motherboard has 2 slots that can utilize the 5 times faster M.2 NVME SSD drives.   These M.2 NVME SSD drives are tiny, generate less heat, and lay flat on the board.    This build has both slots being used to replace the larger SATA SSD Drive 1 and SATA Storage Drive 2.    You would still be able to add your spare 500gb SSD when needed.    In a small case like the Bolt X, space is limited, and heat must be managed...so going to M.2 NVME SSD’s is a win-win on both topics!    See what you think.

Bolt X build2. ($3,650)....Config # 2393979

Just to give options to consider, I also built much the same rig in the excellent mid-tower sized Lumos case where there’s more space and lots more cooling capacity for hot components.    Left out the GPU as with the Bolt X builds.   Very showy case with all Corsair LL Series RGB fans....It also has a better Corsair 850w PSU, a better HydroLux Pro CPU cooler (required to use your preferred hard line tubing choice in this build), the normal ATX sized Asus Strix Z390 -H motherboard ( doesn’t have WiFi onboard...move up to select a MSI board with WiFi if needed).   If you do use the Star Wars Titan GPU, you can ask DS about an extension bracket to display it vertically in the window if you like. Anything is possible with DS if you ask...lol.

Lumos build2 ($3,573)....Config # 2394051