Why would an alternative Senedd have 140 seats (sounds too big - 80-90 seems about right) - but more importantly - why FPTP....
Because it's roughly the cube-root of the population of Wales – it would be in line with the size of large councils and would result in enough backbenchers to keep things interesting.
80 is an arbitrary number that Labour came up with on the basis of a peculiar version of multi-member FPTP, electing 40 male and 40 female AMs from the existing 40 constituencies; and that number's taken on some folkloric character, despite no-one sane believing in that ridiculous electoral system any more.
And FPTP is a perfectly good system that generally leads to strong administrations. I don't see anything wrong with AV or the Schulze Method – it would keep the main parties honest (but we lost that referendum). But multi-member systems in general end up with weird undernomination games (as we see in both parts of Ireland), fudges to mitigate their impact (as in Australia), perverse results (e.g. Lord Bourne), and give undue weight to minor parties (especially those with thinly-spread support) that typically enter coalition negotiations advocating pet policies that are rejected by the voters of both left and right – the electorate deserve better than waiting an entire electoral cycle to have the opportunity to give third parties a well-deserved kicking for implementing lousy undemocratic policies.
I got bored so couldn't resist having a go at this. East Region, 90 seats, 5% tolerence (not sure what tolerence you used?) Only 3 cross-county seats (Beds-Cambs / Essex-Cambs / Norfolk-Suffolk). Only a handful of 3-council seats, and only a couple of orphan wards. Inevitably a couple of compromises but in the circs, not too bad considering! The ward sizes, especially around Basildon, Chelmsford, Southend and Colchester, are not great