Navigation is not my speciallity but IIRC B-RNAV requires 5 NM track keeping accuracy and compliance can be achieved by DME/DME and probably by VOR/DME as well.
That is incorrect, both legally and practically.
Legally, you need a BRNAV compliant nav means which in the GA context can be met only with an IFR approved GPS, or with a BRNAV approved (ancient) KNS80 installation with antenna filters for FM immunity.
Practically, ATC treat all that airspace as RNAV i.e. most of the waypoints you will be given will be either virtual waypoints (not navaid based or navaid-relative referenced) or they will be navaid names but the navaid itself is way out of DOC (e.g. a VOR 200nm away).
There is also no way to navigate using DME/DME - except using an FMS with an INS with DME/DME corrections, which is not GA. Only high-end bizjets and above will have that. Unless you fancy flying your entire route as a series of DME arcs
Anyway that is not sole means GPS, but primary means.
That is complete bollocks. There is no concept of "primary". The regs (for non AOC ops) dictate equipment
carried, not equipment
used. You must be a PPL instructor
When GPS fails you can fall back on alternative systems that don't meet the requirements but at least give you a good sense of where you are.
Actually, in BRNAV-mandatory airspace, you pull out your backup GPS and use that as a DCT box
If you have lost all GPS reception, you advise ATC that you have lost RNAV capability and need VOR-VOR routes, or vectors. This is very rare (GPS is extremely reliable) but it happened to me
once in Italy and I got assigned a different route with a higher MEA (FL160 instead of 140, IIRC).