done this a few times and I’ve had most success in terms of getting money back by buying the cheapest option (lowest spec) for the frame i wanted. The philosophy being
1/ that you always have to mark down your parts by a certain % below CRC / merlin, but 10% lower on a £120 set of brakes is £12, on a £200 set is £20.
2/ at the more budget end of the full bike offering i think pricing is that bit more competitive, because there are more people interested in bikes at around the £1000-£1500 level say than the £2500 level – hence the bike cos. have to compete harder. If you are a genuine enthusiast with £2500-£3000 to spend, chances are you want the ‘Giant’ and just because the parts spec on the ‘Trek’ is slightly better you aren’t going to be as swayed as at the lower price level. Also i guess ‘Giant’ have better pricing for the bigger volumes groupsets of SLX and X7 than XTR and X.0 hence at that level you’re getting your groupset already that bit cheaper than at the higher end.
Where you differ though, of course is where you say some of your kit is getting ‘long in the tooth’ – that said nothing wrong with slapping eg: a deore front mech on for now and then upgrading later. Or use the money you save to upgrade individually anyway. depends how much has to be changed.
best I managed was a Trance from paul’s cycles – effectively swapped out a Sugar onto a brand new frame for a little over £250 out of pocket cost.