A fewe tips:

An inverter with lead acid batteries (especially the mobile units) which cannot control the cutout voltage for the battery (min of 11.5 VDC) is a burden not a solution. If you are subborn and dont want to upgrade to lithium, make sure the inverter is way over spec'd and the has the abilty to adjust the cutout voltage to not less than 11.5VDC ( they will last around for around 3 years).

A lead acid battery in the gate, alarm or electric fence, will only last 3-6 months, they are not designed for stage 6 load shedding. They have trickle chargers for the odd power failure. There are lithium chager/battery replacements and they do work, we have been installing a few of them.

Why do people recommend lithium:

The charge current can be a lot higher, reducing the charge time.

Can drop to a lower SOC, without damaging the battery.

They voltage stays consistant and drops and a much slower rate.

I dont recomend using a lithium drop lead acid drop in battery, I can t say I have had any success with them, it could just be the brand I used. Unless you use a add on lithium charge with a relay, I personally wouldnt sugest it.