Basically none - General election will be in October; none of the parties would want that (especially the Greens who would be heavily squeezed in that scenario I suspect; simply because there are two FPTP elections happening and they'd have to run less constituency candidates in areas) plus the crunch point for a new election will be when the new leader is elected which is now. After that you get to the point where fresh elections are a bit of a waste of time because of the extraordinary election thing.
I think the shift to elections every 5 years is particularly bad for local authorities, especially ones which are all elected in one go, because it decreases democratic engagement significantly. It also doesn't help that Scottish local authorities hold by-elections by AV and ordinary elections by STV, so too many by-elections can warp the partisan balance somewhat.
Honestly the simplest solution is probably four year terms, but postpone the election by a month if it would otherwise clash with a general election.
I think this is right - and would also do the same with clashes to a Scottish election as well. I guess the challenge is that elections are planned fairly far in advance and needing to have the contingency to have counting staff/venues etc in June just in case might also put some people off. Not unprecedented though - in 1983, 1987 and 2017 we had June elections called after locals and in 1992 the General election was the month before the locals (and in 2001 all elections were delayed a month because of Foot and Mouth).
I would shift to 4 years and either fill vacancies via count back (which would result in more candidates standing and possibly a bit of intra-party competition plus also reflect who the next choice would have been) or co-option for party vacancies to try and mitigate the issues when minority parties stand down. Neither option is perfect but I think its better than the status quo.