Unfortunately, I’ve only read Shimeji Simulation in English so I can’t really comment on its difficulty… but I just checked and actually WK has had a book club regarding the series semi-recently.. It’s listed as beginner but it definitely seems like people have had trouble with comprehension due to how bizarre the series can get so yeah might be worth shelving until you feel a bit more prepared for it.
As for no furigana I think the biggest effect of that is honestly it just takes you a bit longer to look up words you don’t know. Which can definitely end up causing fatigue to accumulate but it’s definitely more tedium than challenge!