HI Emma and welcome to The Tortoise Table! Love the name Kimchi and I love Horsfields (but I'm a bit prejudiced there because I keep Horsfields and they are a species with a great personality. I've just re-read this message before posting and I do apologise for the length in advance!!
His weight at 3 - 4 years sounds absolutely fine. We aim for tortoises to gain an average of between 1g - 3g on average each month. So if he was, say, 16g when he hatched, and is now 3 and half years old (42 months) then 42 x 3g per month = 126g + birth weight of 16g = 142g.
So if anything he is a tiny bit more than one might suspect.
The thing about many tortoises, but especially Horsfields, is that they must grow slowly or they run the risk of developing deformed shells. In the wild they live in areas where it is very hot in summer and much of the vegetation dies, so there is competition with other animals for what is left, and tortoises are designed to grow slowly. Horsfields are particularly interesting because the areas they come from tend to have very long cold winters and very long hot summers and field studies have shown that they hibernate in the winter and aestivate (like hibernation but in the heat) and in fact are only awake for about three months a year and aren't even eating for much of that time. So they are sort of programmed to 'eat for England' in the time they are allowed, and consequently in captivity, where we present them with lovely meals every day, they overeat and can develop pyraminding of their shells and metabolic bone disease. You therefore need to monitor his weight gains and if he is gaining on average more than about 3g per month, then cut down on the amount of food you are giving him.
Re the substrate -- I'm glad you're getting rid of that pellet-like substrate. And you are right that Horsfields are burrowing species, and they like nothing better than to bury themselves completely (quite alarming the first time you go to look at them and you think they have somehow escaped from their enclosure!). Your coir/soil mix is probably fine (I'm not familiar with it), but the best substrate is one that replicates what they have in the wild, which is a sandy soil. So the easiest thing to use is ordinary topsoil and children's play sand (in must be children's play sand and not builder's sand), and you can buy bags of these at garden centres and DIY places like Homebase. We just mix them 50/50 (or mine prefer about 60% or 70% topsoil and 30% or 40% sand, and then you spray it every day or two to keep it from getting dusty. What I do is to pour some water on it once or twice a week and mix it up well so that the result is the teeny tiniest bit damp (not wet at all), so that a handful of soil will just start to clump together if you have some in your hand and make a fist. I would have the soil mixture in his sleeping area too, and in general you want it as deep as your enclosure will allow (at least 3" or 4" if possible, so that he can bury himself in it. No need to switch to that now -- the mixture you bought will probably be fine, but you can think about switching to a sand/soil substrate when that one is used up.
The flowerpot is a good idea, and in fact tortoises get bored if they can see from one end of their enclosure without interruption, so plants, rocks, hides, etc. are a good idea. If you do put plants in there, be aware that if he likes them he might just eat them to the ground in no time. I grow decorative grasses and spider plants, both of which mine ignore for quite a while and then one day I come home and they have eaten them to the ground. For extra exercise and stimulation, you could consider (if the size of your table allows it, to put in a second level for him to climb up to. I have a second level in my table with a ramp leading up to it, and I feed them up there, so mine are always running up and down the ramp to see if there are any treats there for them. It doesn't need to be big (mine is too big), and underneath that level can function as a sleeping area. I've attached a photo of mine to give you an idea of what I mean.
Mine have never touched those hard calcium blocks (I think they are just too hard for them), but they will nibble on cuttlefish, and Nutrobal is an excellent supplement (do follow the instructions though -- I think it is 'one pinch per kilo of tortoise', which is not helpful at all when you have a small tortoises. You can also buy food grade calcium carbonate powder that you can sprinkle on food for them on days when they don't have the Nutrobal (they do need a lot of calcium).
You don't need a heat pad, unless your room temperature drops below about 13C or 14C at night. If your thermometers are mounted on a wall of the table then they won't be giving you an accurate reading of the temperature directly under heat bulb, which will be at least 5 degrees, maybe more, higher. You want to measure the temperature directly under the lamp and at the height of the tortoise's shell. I use one of the digital fridge-freezer thermometers with a display unit that sits outside the table and probe on the end of a long cord that you can hang down into the table next to the circle of light -- something like this
https://www.pharmacy-equipment.co.uk/pr ... er-tmm105/ You can get them in lots of places, but it's good to have one that has a Max/Min facility, so that you can see how hot or how cold it got since you last looked, which is really helpful on cold nights, etc. Your thermometer on the wall at the cool end is probably fine, but the hot end might need more accurate monitoring.
There is probably no need to take him to a vet at the moment, but if you do then it's really important to use an 'exotics' vet (they specialise in reptiles, birds etc).. Ordinary vets have very little training in reptiles and there can be problems. We have a list of recommended vets, organised by county, here:
https://www.thetortoisetable.org.uk/tor ... s-by-area/
It's good that you are soaking him, as lights indoors can be very drying. I definitely wouldn't hibernate him this year, as you have only just got him, but Horsfields have a strong hibernation instinct, so if he starts to get lethargic and not wanting to wake up in the mornings and loses his appetite, let us know and we can give you tips to get him going again. You can hibernate him next year, and we have really good instructions for preparing him for hibernation and the hibernation itself that we can send you when you're ready.
I am so sorry that this message is so long! If you're still awake after having trawled through all I've written, here is a link to a care sheet for Horsfields, and do let me know if you have any more questions or problems.
https://www.tortoise-protection-group.o ... 014New.pdf
I think you're made a great start and are going to be a really excellent tortoise keeper, and Kimchi has been very lucky to have found a home with you.
Nina
