I am trying to see if it is possible to have a game where the outdoors encounters are vanilla-like (rare) and the indoor encounters (caves, ruins, forts) are more hardcoe like FCOM.

Is this possible and what combination of mods would I use to accomplish this?

Goal is to have a low stress outdoor experience enjoying travel to destination and a hardcoe dungeon-crawl.

I assume you are using FCOM from your statement.
IIRC, FCOM has a couple of plugin choices for Safer Roads and Less Wilderness Life. I think these will do what you want.

If not, posting your load order would be appropriate.

My latest build has OOO, ROM, TIE, and MMM with spawn rates reduced reduced plugin. I used to play FCOM but have a stabler game with what I have now (3 CTDs in 80 hours of play with sessions as long as 7 hours - and I did not even use the 50 steps to crashfree Oblivion!). Right now I am just looking for a mods suggestion to achieve what I want and start a new game.

I guess my biggest hurdle is not understanding spawns and how they relate to outdoors/indoors.

Are there spawn points in dungeons (caves, ruins, forts) or is that content static in mods? If I use the reduced spawns to something like vanilla, does this affect indoor spwans if they exist?

I just want to reduce outdoor encounters and keep or increase the difficulty of the indoors encounters.

Again, I'm not sure, but I think OOO adds spawn points and changes many vanilla spawns using several different leveled lists.
Meaning that in vanilla, you may get a single enemy. Within OOO, that same spawn may generate several enemies.
Since you're using OOO, you probably would get a clearer response in the OOO thread.

That would be, but I am not married to OOO and would try something else if it is out there that would give me the game I want. Only easy way I know is to keep the game vanilla without overhauls and find mods that only deal with indoors (like dungeon quests). But that is pretty limited and can be played out pretty quickly so I am trying to find an overhaul that does what I want, if such exists.