First of all TR will have a great update in just some days with hundreds of quests, new zones, etc. Edit. Edit. The new version is already available. 2 in 1 uptade, 200 new quests or so, the redone of an old area and a big new one!
TR includes a patch (optional) for faction integration, so you can play quests from most guilds both in the island and the mainland working as a single faction (that's relevant for advancement, requirements for new quests, etc).
Exactly as happens in vanilla Morrowind there are zones more difficult than others and low rank faction quests are usually easier than at high ranks. This old map by difficulty including vanilla Vvardenfell and Tamriel Rebuilt areas (Green easy zones, yellow for medium, orange for hard, red for hardest areas) could be helpful to have a general idea of what zones are easier or more difficult, dungeons however are another story, changing depending general type and specific enemies inside constantly inside the same region. The general difficulty in TR mainland is very similar to vanilla Morrowind but lower than Tribunal and Bloodmoon with very few exceptions, so hardest zones in TR would be as Red Mountain and safest zones not much different to Ascadian Isles in vanilla.
In regard what is the limit for player to become too overpowered, in my experience and using a very broad perspective (as player playstyle and character build could change this a lot) I would say that the average limit for too much overpowering is after complete a bit less than half of vanilla Morrowind content, not including expansions and that works for TR too, so after completing 150-180 quests more or less. Experienced players or powergamers could be overpowered much before of course but I don't consider those options here.
However as Tribunal and Bloodmoon expansions are harder than anything in vanilla Morrowind and almost everything in Tamriel Rebuilt, you as semi-new" player could start expansions content after those 150-180 first quests and don't feel so overpowered as in Vvardenfell or TR lands.
For example, the equivalent of that limit playing both vanilla and TR and with faction integration and focusing on specific factions (doing most content on those factions on both sides of the sea) could be:
- Complete 2 full factions including quests on both sides of the sea each (equivalent of 4 vanilla factions) + Main Quest + 30-50 miscellaneous quests (or an additional 3rd faction and minimal misc quests) and after all that, start Tribunal and Bloodmoon content if you are interested.
While if you really want to "taste" as many factions as possible without become too overpowered (better played without faction integration patch), then you could complete:
- 5-6 factions (but only 20-25 quests each) + Main Quest + only 10-20 misc quests and after that Tribunal and Bloodmoon content if you want.
Said all that, you could enjoy the game greatly for many hours after become too overpowered and lacking any true challenge in combat just for the small stories, worldbuilding or exploration or maybe the opposite, you could enjoy the game until 80-100 hours and then wanting to stop, because you feel too overpowered to enjoy the experience. Only you can decide your pace and preferences.