A second new start

Good morning to everyone from Greece,

My name is Chris.First of all I would like to give you my congratulations to your very well organized and usefull forum for the discus keepers.
After reading carefully all the information about keeping discus that are provided here, I realized that the parameters and "rules" in keeping discus are not exactly
the same worldwide.

I cannot describe myself as a beginner in keeping discus, but also not so much experienced, taking into account that I have kept them in my tank from 2011 until 2017 (I bought 5 discus in 2011 but 4 survived until 2017).
Now, I would like to make a second, more studied and organized, start. I'm in the organizing phase right now. So, any suggestions and remarks from experienced discus
keepers may be proved crucial and important for my new discus project.

The description of my tank:

1. internal dimentions: 107cm x 42cm x 50cm (LxWxH) Open. Max actual capacity around 60 gallons. The real capacity of my tank, taking into account the aquascape is 50 gallons.
2. Substrate: 0.95" sand with plant nutrients (dennerle company) and 0.95" gravel (0.3-0.5 mm size). Sum 1.9".
3. Filtration: Two external filters. Tetra EX800 plus (https://www.tetra.net/en-gb/products...nal-filter-set) and Tetra EX1200 plus
(https://www.tetra.net/en-gb/products...nal-filter-set)
4. Plants (tropica company): Anubias barteri x 3, Echinodorus Bleheri x 2, Hygrophila Polysperma 'Rosanervig', Vallisneria americana 'Natans' x 5, Staurogyne repens (a small
bush), Spiky Moss Taxiphyllum.
5. Heating: 300w (aquael platinum), 200w (Dolphin)
6. Lighting: 2 x 54 W T5. Sum 108 W.

My Water Parameters:

1. 100% RO water with added liquid minerals (Seachem company): Discus Trace, Potassium, Flourish Advanced
2. NH4/NH3 & NO2: 0
3. NO3: 5-10 mg/l
4. Ph: 6.5
5. kH: 3-4 dKH
6. gH: 5-6 dGh
7. Temperature: 84.2 F

All the above mentioned water parameters have been measured using sera liquid tests.

Water changes:

1. Now 50% once per week.
2. Planned (after the introduction of discus) 40% every next day

Inhabitants of my tank: 12 corydoras sterbai and 4 rummy nose tetras

I'm planning to buy 4 or 5 or 6 3" discus next week. So, the questions I would like to pose are:
1. The schedule of my planned water changes is OK?
2. How many discus do you recommend to introduce 4, 5 or 6? (Taking also into consideration the inhabitants that already live in my tank)
3. Is it necessary to remove some of my inhabitants?
4. What must be the ideal (max) concentration of Nitrate (NO3) for my water in order to reorganize the schedule of my water changes?
5. Another issue that worries me is that my LFS has discus for sale imported from Thailand only. Is there any problem experienced from keeping Thai discus?

Thank you in advance for your answers and suggestions.