Thanks, TBH it all feels a bit slow, but then suddenly a part gets finished!
The underback will be pumped and include a PID-controlled element, so step mashing would certainly be possible. The MT is fairly wide diameter, so the grain bed shouldn't become too deep and compacted for a reasonable recirculation speed. I may also include a grainfather-style overflow to further control this. But that said.. my intention is more generally to use it for tweaking and maintaining temperature than making steps as such. Most beers I make are British styles, so mash steps aren't of any great benefit, and at these volumes would take quite a lot of power for a good ramp speed.
However, when making (say) a hefeweizen then I certainly do want a ferulic acid rest with a subsequent (and quite large) step. In this case I'll mash in thicker, then achieve most of the step by pumping in more pre-heated liquor from the HLT, before then tweaking with the underback recirculation (a kind of hybrid approach, I guess). The MT will be 100L so should be reasonably flexible in this respect, especially if not being used for very high gravity beers at maximum brew lengths.
The kettle works pretty well without the lid, if both elements (total 5kw) are running, but in the current situation (at least in winter) steam is a problem so i may need to duct it, in which case I shall use a lid with a kind of chimney. I've no qualms if that is so; DMS is much more volatile than water and isn't going to condense easily on a steam-heated lid provided there is sufficient chimney diameter to allow easy escape; this was certainly so with a previous kettle. Boil-overs can be a problem with partially covering though, so I'd have to juggle number of elements and chimney diameter. There are ways of controlling element power but I'd hope they weren't necessary.