All that means that you wouldn't choose Ammonia as main fuel intentionally.
You anyway have to produce Ammonia in great amounts to make any type of a storable propellant.
And you have a lot of the storable propellants to get out from your base to any other place.
So, unlike Methane, you need Ammonia daily. And as you anyway are producing it in great amounts, why not use it also as is, avoiding Methane at all.
You have just three resources on Mars: CO2 N2, and H2O.
And while CO2 and N2 are everywhere, H2O is located just in very special places, in glaciers, not across all over the planet.
And H2O is the only hydrogen on Mars,
So, either you are nailed to a glacier and can't get nowhere from there, or you have to deliver hydrogen from glaciers.
Options are: liquid hydrogen (highly cryogenic, highly unstorable, requires a lot of equipment mass to hold it for weeks) or chemical compounds rich with hydrogen (in fact, just 3 options: H2O, CH4, NH3)..
CH4 contains H in ratio 1:4, but is highly cryogenic, so badly storable, needs cryogenic equipment and electric power, and you have to transport mostly carbon (which you have everywhere, so it's a ballast).
H2O is absolutely storable, and is not cryogenic. But it has just 1:9 of hydrogen, and O is a ballast, too (like C it's everywhere as CO2).
NH3 on Mars has a "room temperature", most part of time it's liquid without heating or cooling, it contains H in 1:5.7 ratio. And it contains no ballast, exactly because nitrogen is relatively rare
So, placing ammonia plants on glaciers, you have to deliver only precious things: H and N. Everything you carry to your base is highly useful and required in great amounts.
Methane doesn't make sense even there.
Yes, you have a lot of CO2 to produce methane (if you have water right where you are or have delivered, say, ammonia from a glacier factory).
But how much methane can you store, as you need cryostats for this?
How can you use it except fueling a big return rocket?
Methalox is a very poor choice for anything but a big dumb rocket. No hoppers, no fliers, no drones. No camps with ready-to-use fuel tanks.
You even can't get somewhere for more than 1-2 days if you fuel your hopper with methalox, you can just drop down, plant flag, take a bucket of stones, and quickly get away.
You want to use CO2 which is everywhere. Easily.
CO2 + H2 → CO + H2O
CO + 2H2 → CH3OH aka methanol.
(Of course, you need given temperature, pressure, and catalyst, which are well known).
Methanol is as storable as water, as Universe. It's stable, it's not cryogenic.
To make UDMH or that bad and poor MMH, you use ammonia and methanol (the latter even in two places of process: to produce methylamines and hydrogen peroxide for a modified process).
To make polymers, you don't need methane, you can produce olefins ethylene and propylene right from methanol.
To produce aceton for HTP and hydrazines, and ethanolamines for purifiers, you anyway need methanol.
To mine the frozen ground you need solutions containing either methanol itself, or anti-freeze agents made of it.
OK, you need methalox to fuel the return rocket...
Wait, are you going to send people onto another planet even with no abort option?
And how should the rocket land? It should carry cryogeinc tanks for 8 months of flight or hope that they target enough accurately to get into aerobrake corridor right from fly-by?
So, the return rocket should carry storable propellants onboard. Hypergolics are the only choice.
So, even for this methane makes no sense.
And, as you need a lot of methanol and ammonia in any case except your base is just for flag planting,
why add another expensive technology (Sabatier is just the first stage of the process)? On ISS they don't care if it''s enough pure, they just vent it out.
You need some primitive and raw fuel in great amounts?
You are anyway producing ammonia, it has ISPg 3200, it doesn't need heavy tanks, coolers, and energy.
Just take it more or (even better) use a tripropellant engine (like RD-701, but for ammonia instead of kerosene; like RD-301, but with hydrogen and oxygen instead of fluorine; both were made in metal).
Tripropellant engine is hard to make? Well, at least once it was (and is, but wasn't required).
And hard compared to what? Interplanetary landing ship with ISRU fueling? Seriously?
In fact, the only place you need ammonia as fuel is ammonia delivery from glacier plants.
In this case this is absolutely obvious choice of fuel.
For a return rocket you anyway need hypergolics (made of ammonia which you have brought from a glacier in an ammonia-fueled rocket).
Methane is up to -160°C. Mars is -100..+30°C.
Of course LNG tankers can store it. But what is their mass-to-gas ratio? Is it 9 t of gas per 1 t of ship?
You can produce and store methanol as much as much hydrogen you have. (Exactly like methane).
Its liquid temperature range is -97..+64 °C. So, it's always liquid on Mars, just have enough barrels.
You can store it in in your research camp as much as you want, it won't boil off.
So, you can use it as the cheapest possible propellant between the storable fluids.
But as you anyway need hydrazines, you can just enforce it with hydrazine hydrate admixture (historically - C-Stoff).
So, C-Stoff (methanol+hydrazine hydrate) + hydrogen peroxide is the most cheap and handy solution for ground equipment when you need a lot of power at once, from time to time.
Without nukes on heavy rovers.
(Maybe C-Stoff can be used with HNO3 or N2O4, I don't know. In this case it would be even cheaper.)
Also you anyway need explosives for mining, geology, and landscape shaping.
The obvious choice is astrolite (made of the same hydrazine hydrate and oxidized ammonia).
You need some kind of primitive plastic envelopes for charges and sand bags for henges and rad-protection.
You can make it of methanol (methanol → ethylene + propylene → polyethylene and polypropylene). Of course, of poor quality, but "quality" for sandbags?
So, methanol and ammonia are the blood of Martian base.
Use them as widely as possible, as you anyway have to produce a lot of them.
While methane is useless.
(Methane gets useful much later, when you build an industrial base and deliver methanol and ammonis to a huge orbital industrial base.
Then of course you just launch uncrewed superheavy reusable Nexus-like shuttle rockets from glaciers and fuel them with methane.)