I checked that link and a few of his apps. I don't know if he can live from it when most of them had 10-50K installs and you don't know if people still play them.
Anything is still better than nothing
I don't want to discourage you but when you dream of going full time with a small office and all that then the math part of my gray mass is starting to rattle.
I don't know your netto wage and I don't want to know it since it varies too much from country to country and living near a city or not
but let's take an average euro netto wage of €1500/month which is for some much for some too less.
so you'll need to build up 1500x12 that's 18.000.
added to what I expect that would be 36.000 to build up the wage+1 year backup (unless you built it up now during your daily work)
when you make a static average of €20/day from your apps which is already good that means 600/month. which is a bit more than 1/3 of the aim.
so you'll need 30 months to cover the basic year wage which is 2.5 years instead of 1.
doing 3 other invaders clones might increase/double/tripple earnings but on the other hand people might just move on to the latest version and forget the rest
so worst case you'll stay static or even drop in earning.
more apps is more chances to earn altho a lot of us didn't experience this at all and only have a few that actually are worth mentioning the earnings.
if the app doesn't have visibility or doesn't pass some good worth-to-mouth you'll be stuck with no installs and nearly no income from that app.
it's all so unpredictable.
so 6 good earning apps of €20/day would mean 20x6=120 > 120x30=3600 which is more like it and 'seems' to be enough to live from and build up the extra year.
now that you will be self employed you also have some duties to follow.
quarterly tax sheets require you to list your income, expenses and you'll be taxed on the profit you will make.
for example 3*3600=10.800 depending on this sum you'll lose 20-50+% on taxes depending on the country and taxating level based on the income from the previous year.
extra costs for social security
extra taxing for your 'office' even when working in the house you already bought and already pay commune/province taxes for already (overhere 25m² minimum)
province/county tax.
if you pay out yourself a real wage you'll lose again on personal taxing and need to pay part of the social stuff too for your 'employee' which is yourself and the other part as receiving employee.
some additional stuff like accountant, internet & mobile subscriptions, apple dev account, b4x renewals etc can be added as expenses.
+ some stuff I probably forgot about...
so in the end you'll be happy if there's still 5.000 left of that quarter after paying back part of your income for taxes and all that other stuff.
and then I'm not even talking about the rented office idea you have in mind which is probably €800+/month extra cost.
ok, it will lower your profit so the taxing value will drop aswell but I guess you better start working at home first to see how it goes
unless you really want to escape from your wife during the day
a maybe country specific side note...
overhere you can start earning with the internet/apps or anything else and add it to your personal tax sheet as secundary or additional income.
they will 'only' tax you on 50% of that value.
unfortunately our tax friends are smart and they set a limit on this. if they notice this is getting big they'll force you to become self employed (full or as second job)
so they can tax you even more.
maybe there are options to work part time (1/2 , 4/5 ...) so that you have more time for your game coding?
then you still have your guaranteed steady income.