Ideas to gas heat & DHW a 450 ft apartment with moderate cost and maintenance?

So I’m finally going to add real heat to my last ‘gas on gas’ apartment, on floor 3 of 4 in a 120 year old brick, uninsulated ‘tenement’. These 450 sq ft units were built as cold water flats. Previously tried 2 decades ago was a Minitherm boiler with an indirect tank: big footprint, clanky to have in your living room, and often needed resetting due to some venturi effect from being on the top floor. I pulled it out.

Installed now are 4 systems based on a 40 gallon DWH with a bronze circulator pumping 140 deg water through hydronic baseboard. Several years ago in one top floor unit I supplemented the radiation with 2 kickspace heaters. That one has a total radiation @140deg of ~18k btu, the others ~9k. A 6th unit was converted to the upper floor of my own unit, with total peak radiation of ~13k off my 180 deg boiler, but it’s zone cycling is fairly low.

So far the biggest downside to these is potential water damage from tank failure if the pan and leak detector device fail. Maintenance is very low. But I’m open to new ideas, for this last one and the others as the tanks fail.

Heating Contraints:
Can’t vent through the brick side wall, it’s on the property line. I have a huge flue for the original coal burning stoves on each floor.

I have a ¾” gas pipe running 55’ from the basement meter to where the heat will be.

I do not have a drain where the heat will be for condensate, it would have to pump up, through the ceiling over the bathroom and down to the kitchen drain somehow.

Questions:
I’ve been told I should be using a heat exchanger on the current systems because of Legionella risks (local contractors & vendors poo-poo this). So if I am simply reproducing that system, any advice on this improvement would be useful. I was looking at simple exchange blocks, but they have a temp drop rating that concerns me since I’m already at low btu throughput. Or is that irrelevant due to the low emittance relative to the heater btu? That once it’s circulating the block can maintain high temp transfer since it’s being asked for relatively little ∆T?

Other than that, it looks straightforward. The rad loop system is set up like a boiler’s with a circulator, pressure reducing valve and an expansion tank with an air scoop. The DHW side is like I already have, just needs a circulator loop with check valves and an air vent, and that one runs 24/7 year round to not let the water in the loop cool and stagnate.

I'm considering doing away with the baseboard in this one and just using kickspace heaters in wall cabinets. It's easy, and in a tiny apartment the baseboards reduce usable space. Downsides?

I don't think this is a good radiant candidate, in such a small space, there isn't that much unobstructed floor.

A mod-con system is fascinating, but many of it’s virtues do not hold up with a rental. The efficiency vs maintenance seems to make it less appealing to me, even my plumber agreed after some discussion. The difficulty of venting is another problem. I’d need to drop the vent and air pipe 20 ft from the roof through the flue and somehow connect to it through the brick flue wall. It would be a trick getting rid of the condensate, and I’m not even sure I have enough gas for the on-demand DHW. The unit itself is 3-6x the cost of a 40gal atmospheric vent DWH. Is there something I’m missing?

Speaking of water heaters, can anyone explain the advantage of the ones with side taps for space heating? I have some with, and some without. I’m not talking about the ones with an internal heat exchanger, just the side taps.

I know this is overlong, but I didn’t want to make like 4 different threads. Any advice is welcome! Thanks for reading this far.