I'm in a 57M GP guild. We can do tier V but now only do tier 2 because the rate at which we get guild currency and credits from that is higher, and we consider the gear rewards from tier 1-5 essentially worthless, especially compared to the time investment.
I can't comment too much on the teams, we just use a variety of whatever we have laying around because the rewards for anything less than heroic isn't worth building teams for.
Sion and traya I find pretty enjoyable, I find the interactions and decision making (should I cleanse the pain/bonds or squeeze out some extra damage? When to topple?) and the variety of teams that can be used to be fairly enjoyable to figure out.
Nihilus is awful. It takes forever, almost no teams are good against him, and the unique ability isnt really fun or interesting ( eyeball the turnmeter and hope you guess the timing right :/). The decisions (special vs basic, use unbreakable or not) aren't interesting or meaningful because they are essentially forced. This makes defeating nihilus more of a boring chore than anything else. Despite this, I think we could get used to the chore if the reward was worth it.
The mechanics and health pool of this raid essentially require very heavy participation from almost every member, every time. It is very frustrating to spend so much more time and effort just to eek out 5 percent or so in a phase, and be rewarded the equivalent of 1-2 guild activity rewards and a few credit daily challenges. I do like the extra source of guild currency and credits, but the ratio of effort to reward feels bad. The rewards don't do much to help ease the gear crunch or progress towards the next tier, and there is no real incentive to try a harder tier until you are ready for heroic.
To give non-heroic guilds a way to enjoy the STR, I think it would be a significant improvement to add useful and difficult to acquire items to the lower tiers. Maybe the lowest tiers can offer significantly more credits and a mix of purple/omega mats. Perhaps another tier up rewards a set of good mods and a increase in GET, perhaps another tier includes a zeta for good finishes or good chances to drop full or half pieces of guns/cuffs/markV droid callers and other good gear, maybe a tier offers some crystals and rancor/HAAT gear (in a way that we don't have to beat the raid 9 times to get a full piece). Maybe one tier could even also award vet smuggler shards since the RJT team is apparently the only good team for phase 1.
There should also be some decent incentive to hit top 10 instead of 20 or top 20 instead of 30 etc if you want to encourage participation from everyone. If I can get basically the same reward for 25 as I can for top 10, I'm not putting in the extra hour(s) this chore of a raid requires. I like the idea of lifting the whole guild, but some layers of incentive are still important to give people a rewarding goal to work toward.
TLDR: I'm honestly not entirely sure if the play of the raid is really a problem (although nihilus is boring), but the rewards compared to the effort required is definitely a problem for guilds that aren't close to beating heroic.