N+1 mainly for turbo

Hey all. Been awhile.
Considering another bike, I’ve lost the weight (from 100kg to 68kg) built my fitness, on and off the bike, now regularly running too, I need/enjoy cardio and the mix of the two. I also do a bit of s&c. Anyway for various reasons, having a home gym built, already got Treadmill and weights and be adding a DD turbo to it. To start with I’ll just use my one and only very good do it all bike but will get annoying swapping it around all the time, so want a second bike, mainly to use on turbo but occasionally could ride outside depending what I’m doing.

I currently have a Mason Bokeh adventure bike; Aluminium frame, carbon forks and 3T gravel bars, mechanical Ultegra disc, Hunt wheels with 32G1 tubeless wheels, used for commuting (Gard’s/rack/lights semi permanent hope R4 system/SPDs) adventure/sportive/endurance all day/touring/off road…

I’m not sure what second bike to get to compliment it, I think full carbon (only had aluminium bikes with carbon forks and now bars) won’t need to fit guards/rack, can be rim or disc, 105 as min or equivalent, ultegra or chorus, cycle to work scheme, ideally cheap as possible, second hand (appreciate wouldn’t be able to get on CTW)
I wouldn’t mind buying bits and building up Inc stripping and resprying frame but don’t have bits sitting around or all tools so probably more expensive to do this.
All bits on current bike work fine and only replace when worn out, so nothing to take off this and put on frame,. No upgrades needed, although, I wouldn’t mind “upgrading” from Ultegra to Chorus on the Mason. Although wouldn’t make it any faster, I don’t plan to race but maybe start going on chaingangs faster group rides and TT albeit more likely to “race” indoors. Maybe hill climb, also staying with at least same brand groupset easier to maintain at home.

Basically keeping the Mason as the utility/practical good bike what cheaper carbon second bike would compliment it?
Mainly for indoor Zwift as well as longer sessions (alth guess could still swap between the two just not all time) so maybe a faster/aero/lightweight hill climbing bike (yes I know it’s all.in legs and could still.lose a bit more weight) more road focused fast fun bike?
Thanks Andy