As you’ve noticed, the taxes, fees and charges on even Economy redemptions mean that the best value is to redeem for CW or F. A cheap economy ticket to, say, JFK or BOS is about the same price or only marginally more than the TFC on a redemption - and earns, rather than burns, Avios. For this reason, spending Avios on Economy redemptions usually isn’t good value (exceptions sometimes exist, eg if you’re very Avios-rich, or are traveling at short notice/another time when cash fare are very high - in these situations, an economy redemption might make sense).

Not sure about specifically “low tax” routes but by starting in INV or JER, you can avoid paying about £175 per person in air passenger duty. By way of an example, the TFC for INV-LHR-BNA (and return) was £524.82 per person when I booked it in November for Club World; it would’ve been another ~£175 each without starting from INV.

You cannot do an ex-EU using the 2-4-1: you must depart from a UK/Channel Islands airport.