I strongly recommend a FPR Himalaya. They are great pens with ebonite feeds that come in both #6 and #5 (5.5 as FPR calls it) nib sizes. FPR have both flex and "ultraflex" nibs, I recommend getting at least one of each nib, so can see which one you prefer. Personally I love my Himalayas.
FPR have some other pens that can house their flex nibs, such as the Triveni which can accept standard international ink cartridges, but my most positive experiences have been with the Himalayas. Their ebonite feeds can keep up with the nibs in a way that plastic feeds can't always seem to manage.
I love my Noodler's Creaper, but I don't use it because it dries out too quickly. As do Noodler's other "vegetal resin" pens (same as many other companies' cheaper plastic/resin pens), their acrylic and ebonite pens work fine in my experience but only come in #6 nibs, so if you prefer a #5 (as I do) then the Himalaya is the way to go. If they ever release an ebonite or acrylic Creaper I will grab one of course.
BTW it seems Osprey have a flex nib now (eg https://www.ospreype...osprey-flexpert), that I believe comes in a section (the "flexpert" I guess?) that also accepts some dip nibs. I've only just heard of this one and I plan on picking one up soon, but for now I cannot comment on the quality.
Having said all that, my personaly opinion after experimenting with "affordable" modern flex pens for a couple of years or so is this: flex is fun, but stubs are easier to use, easier to get, less hassle, and writing with them looks great. They don't necessarily produce writing that looks better than flex nibs, but it doesn't look worse,and it's easier to get it to look good than it is to get flex writing to look good.
So yeah, don't neglect stubs.
My first pen I got ever was a Lamy Safari and I didnt care for it much until I tried a stub in the store and it turned a pen I hated into pen that I like a lot. I own a few other stubs as well, however I have to say the stub in the safari i tend to prefer over my others. I also have a stub in a churchman prescriptor, twsbi eco, and metropolitan. I thought I would like the churchman prescriptor stub but i cant pin point what it is about that stub nib that just doesnt feel right to me. Everytime I write with the churchman stub i struggle with it. My stub in the eco i like too, but thus far i much prefer my stub in the safari; it just writes without fail.
I'm still very much in the phase of learning my nib preferences. I got into the hobby back in 2015 or there abouts and about a year or so in I splurged for my first big purchase and got a lamy 2000 with a fine nib. At the moment it's probably my favorite pen in my collection. The moment I first inked it up and started writing I was like oh my god this writes amazing. Unfortunately since then I also have the thought, hmm I wonder what else is out there lol.
Back to stubs; yes when i first got turned onto them I went and purchased replacement stubs for all my pens; however the stub in my churchman feels persnickety compared to my safari stub and eco stub. Anyhow yeah i do like stubs too.