Trials XP gain and potential tedium

1. It's been a while since I played the game without trials, but if I remember right, you'll need a full completionist run of the base game + at least 1 DLC completed (Descent or JoH since Trespasser needs to be played last) to reach level 27.

2. I love Even Ground and use it pretty much 100% of the time now because it allows me to tackle regions and main story missions in the order I want without having to worry about being over-leveled. I don't find it tedious at all, unlike Walk Softly, which I also find annoying due to the endlessly regenerating barriers and guard. Farming animals for materials takes a bit longer, which is annoying but not overly so. I haven't noticed any more HP sponge with Even Ground on than off. I wouldn't say it makes the game much harder, it just changes how I do things.

3. As I said, I leave Even Ground on all the time. Then I usually turn on Take It Slow around level 14-15, once I've got some of my favorite skills (I'm too impatient for skill points to use it early in the game). I also use some of the other trials periodically (mostly Walk Softly, Travel Light, Grizzly End). I reached level cap about halfway through Jaws of Hakkon (having completed everything in the base game + Descent beforehand). I've never completed the game with Take It Slow enabled at all times, but I assume you can still reach level cap before the end. JoH and Trespasser reward a lot of XP. 

Even Ground is a great trial imo, the fights are bit more challenging, which is nice, if you already know the game mechanics. I turned off Walk Softly after a while because it gets boring, especially in the Western Approach with all the barriers, you have to destroy for the main quest there, that something simple like those barriers take so long.
I often use Grizzly End to make the bears a bit more challenging. But be careful, I get killed at Storm Coast, in the part with the river, that leads to the cave with the spiders and the rift, because there it isn't uncommon, that three or four Grizzlys spawn at once  - that was a bit hard, especially with a low level group.

I enable Take It Slow as I need. If I see, that I level too fast, I turn it on, after a while , I switch it off again. That way, the levelling is balanced for me

I always enable Take it Slow for the whole game until I reach Trespasser, then I switch it off.

Thanks guys on your informative replies. I started a new PT a couple of days ago without Walk Softly cause, yeah, over the top. It's vanilla for now, but I'll turn on Even Ground as soon as I start encountering underleveled opposition in any significant numbers. As per PapaCharlie's recommendation, i'll enable Take it Slow after Skyhold (which I usually reach after Hinterlands minus SW corner + Fallow Mire + Storm Coast).

Should work out nice, I hope!

ETA: In response to PapaCharlie's question on chronology, it should work something like this after Skyhold: Crestwood - Western Approach - Adamant/Fade - Exalted Plains - Emerald Graves - Orlesian Court Main Mission - Hissing Wastes - Emprise du Lion - Arbor Wilds...etc.  As for DLCs, I was thinking of doing one of JoH and Descent not long before the end of the main game + the other after the main quest ends. Do you have recommendations, regarding story impact and combat difficulty (and Solas, if he has any particularly noteworthy stuff), which one would be better to do first? 

In that case, you'll probably hit the level cap at or before Doom Upon The World, without Take It Slow. With Take It Slow, probably not until you are part way through Trespasser, depending on how much you do in JoH and when.

You can either do the DLCs in release order and do without Solas, or, you can do what I've been recommending recently:

1. Complete What Pride Had Wrought, Inquisitor drinks from the Well
2. The voices tell the Inquisitor to go investigate the earthquakes in the Storm Coast, start the Descent
3. Do the first half of Descent. At the camp right before the final elevator down, the voices tell you to fast travel back to Skyhold
4. Start Jaws of Hakkon. Do the first half, up through What Yet Lingers and acquire the Aegis of the Rift. This is very handy for the second half of Descent.
5. The voices tell you to go back and finish the rest of Descent.
6. The voices tell you to go back and finish the rest of JoH.
7. The voices tell you Coryphish is on the move, complete the rest of the main quest.
8. Trespasser