We have done three trips with REI Adventures and one with MT Sobek ( who also run the National,Geographic and Smithsonian Active Adventures trips). Yes, they are expensive, but not as expensive as Backroads. We think it is worth it for the camaraderie, the convenience, and the local knowledge. But we also do self-guided walking trips (in the U.K.) and totally independent active travel with our extended (mainly in Switzerland which I know quite well).
My sister and her husband did one trip with the Sierra Club (hiking in Picos de Europa in Spain) and will not use them again as the guide was unprofessional and did not handle the group well.
For a less expensive trip, I like the Europe guided walking Holidays offered by HF Holidays, a U.K.-based member cooperative (non-members are welcome). They offer full week lodge-based trips in many European countries, with guided walking at various levels each day. The guides are volunteers and passionate about what they do.
https://www.hfholidays.co.uk/destinations/europe/
The cost is much less than the commercial trips that move from place to place. For example, you can spend a week in the Dolomites for £1148, which includes lodging, all meals, transport to trailheads, and guided walking. (That is the price without airfare from London; they also offer packages with airfare included for a bit more). They still have openings for this summer at many places (including the Dolomites). We were just in Dorset on an HF Holiday walking trip last week, and the guides explained that bookings are down for their continental trips due to "Brexit fear".