Air sealing from underneath is difficult. I’m having similar problems insulating from above to be honest. Spray foam is often the only choice because the labour costs of other strategies are just too high. If anyone has seen good attempts at creating a good continuous air control layer between sleeper walls and joist from beneath, I’d love to see them.
However, in the medium term, it’s my opinion that you can justify replacing the whole floor system, since it allows the floor to be lowered to allow for a hydronic underfloor heating system. This needn’t be for the whole house.
A 65mm build up, from the sub-floor to finished surface, is recommended for this. Underfloor heating is valuable for utilizing low temperature heat to achieve high COPs, and for general comfort levels. There may also be a problem reaching peak heating power output without underfloor heating - especially if the rest of the envelope is not fully retrofitted as part of a longer term phased development.
Furthermore, where are your ducts going? Uncontrolled air exchange across the enclosure dominates thermal losses and comfort complaints. Tightening up the building for high indoor air quality and general efficiency requires mechanical ventilation, often from a central MHRV.
Without an intermediate floor system, attic or crawlspace duct runs are often the only way to get horizontal distribution. Which one will you choose to condition? Because it is just a bad idea to have ducts passing through unconditioned spaces.
The roof requires much more insulation to reach the same levels of performance as the crawl space, because of the larger surface area of the pitched roof and due to radiative cooling at night. However, conditioning the roof is easier to access and has the additional benefit of not needing to air seal the attic ceiling - which is a significant challenge by itself.
However, replacing the base floor system for underfloor heating gives you access to the ground to add the drained granular pad, insulation, membrane and thin screed, all to create what is essentially a mini basement. Which will allow you to run mechanical systems under the floor, including ducts, and you don’t need to worry about insulating between the joists (infact, it’s better for the underfloor heating). This need not be for the whole floor area, but could be applied to a few commonly used rooms, such a living room/dining/kitchen.
In bungalows, due to poor form factors/ SV ratios, it’s either: hydronic under floor, ducted heating with a central return, or actively recirculating wall mounted radiators, which are needed to get good COPs and power output from your heating system. Unless you manage to go all out and get passive house levels of performance out of your building enclosure to reduce demand so much it doesn’t matter - replacing windows, doors, avg 200 mm insulation <1ACH etc.
These are not easy decisions to make at the moment. However, once building surveyors and mortgage providers start taking this seriously, and if very low interest, if not subsidized, loans are made available, this will be the norm. I honestly think we need way more publicly backed financial options to support doing these things properly.
Not sure if this helps either!