There’s a moisture management hierarchy. Prioritize them in roughly this order before worrying about thermal resistance. A phased approach over a long period may be the only way to do this, depending on your budget. Get a very good bricky. High performance retrofits of interior insulation on old masonry mass walls is the hardest thing to do to. It goes a little like this:
Prevent the water getting on your masonry in the first place, through a combination of - larger overhangs (hard to retrofit), better pointing including replacing damaged bricks (they are not created equally, modern bricks are way better, testing may be necessary), brick treatments to repel more water (may need to be re applied more regularly to the parts of the building that see more weather). Setup up a maintenance schedule with said bricky. All this generally increases the durability of the building anyway.
Worry about moisture transport through air leakage before vapour diffusion. Vapour diffusion is orders of magnitude smaller compared to moisture transfer due to air movement. Liquid applied air barriers are okay, but parge coats are better because they flatten the wall for the insulation. This includes making sure your insulation cavity doesn’t have a lot of convective loops going on - transporting moisture on the air from the inside to the condensing surface of the masonry is probably your main concern. I doubt there’ll be much vapour drive inwards if you do this well. Your installer will probably not respect tight-fitting insulation enough, as it’s usually thrown in with reckless abandon.
Think about a vapour throttle/retarder, or smart membrane, before considering a vapour barrier, which is totally closed. I suspect the best place to put a smart vapour throttle would on the internal side of the insulation. These smart membranes make life easier, because they allow drying inwards, put prevent vapour pressure outwards. Maybe the manufacture of the product with have guidelines for how best to use their system in your conditions. At least you’ll know what to ask.
Expect failure and design using moisture resistant materials. Insulation that is both vapour open and moisture resistant. I suspect mineral insulation would be best here (no foil, anywhere). Avoid using untreated timber batons in direct contact with the masonry. I think it’s possible to use rigid insulation fixed to the wall with the batons of the service cavity. I like using 25mm plaster board for sound proofing and thermal mass - as thermal mass needs to be in contact with the conditioned air to be useful to regular diurnal temperatures.
As you can see. This is not easy. I would not take my word for it. The issue with DIY retrofits is that you have to become a bit of an expert. This is why community retrofits, followed by long periods of monitoring, are needed to gain trust and confidence. If you want to be a hero in your community, put sensors all over your enclosure, in the most sensitive areas, and monitor how it goes, to compare with simulation. Some other sod can push that to failure with more thermal resistance another day.
Or use expert design/consultation and get warranties, which costs a lot.
In conclusion, if it were my home, I’d go with exterior wall insulation first, if possible, on as many facades as possible. Then treatment to the masonry to avoid water absorption as much a possible. Followed by an interior parge coat/liquid applied air barrier that acts as a vapour throttle (0.1 to 1 perm) edit, actually, not sure about this, continuous mineral rigid insulation system, smart membrane allowing drying inwards but resisting vapour diffusion outwards, followed by a 35mm insulated service cavity and your plasterboard to maintain integrity of at least one cavity after services are installed. I would say 100mm of insulation in total is about as much as I’d be comfortable risking, even 70mm would be fine. More insulation = more risk of failure.
To reiterate the concern about your floor joist, the vertices and edges joining walls to floor to roof systems etc is where all the detail is most important.
You may also have to consider getting better heat exchangers in the building to use more energy more cost effectively (lower temps = cheaper heat). A ventilation system with a whole house dehumidifier can keep relative humidity down during sensitive parts of the season, no need to go above 35% imo. Thermal energy storage and heat networks are probably our only hope to do this to everyone’s place fast enough, but that’s an entire different discussion.
All a bid daunting, but that’s what it takes.
Might jump into sketchup and post the assembly later. The product research would be interesting, I’m a little out of touch with what’s available these days.