How much would such a sun shade weight? Current solar sails weigh between 3 and 5 grams per m2, so 1g/m2 is probably possible in the near future. That is one ton per km2. The radius of Mars is 6000 km, so the area of the cross-section is about 100 million km2. To obscure a few percent of that would take a few million tons of material. That's a lot, but a million ton is the cargo capacity of 5000 Starships. It's not completely impossible.
Edit: apparently, as per Wikipedia, in the high pressures of Venus, the resulting pure oxygen atmosphere would combust the solid carbon sediment, going straight back to the same carbon dioxide atmosphere is has now.
There are two major concepts that I found really fun: 1) airships and 2) steampunk rovers.
Airships:
2018 JPL: https://science.nasa.gov/resource/aerial-platforms-for-the-s...
2020s report on in-situ sampling using airships as mothership https://kiss.caltech.edu/final_reports/Venus_In_Situ_Final_R...
Clockwork rover for long-lived venus exploration: https://www.jpl.nasa.gov/news/a-clockwork-rover-for-venus/
I don't think that last one was (too) serious, but it was interesting to sus out the costs/benefits at TRL1-2 at least.
AGU is a fairly open, very accessible conference for tracking progress on these. Here's a short paper on venus long lived surface missions.
STRANDBEEST EVOLUTION 2021: https://www.youtube.com/watch?v=C97kMKwZ2-g
Screw Venus, they need to work on adding mechanical computers to these Strandbeests, so they can turn around and wander the world's beaches autonomously for years at a time.
https://nssdc.gsfc.nasa.gov/planetary/planets/venuspage.html
DAVINCI - NASA Venus Flyby and Probe (2029)
VERITAS - NASA Venus Orbiter (2031)
EnVision - ESA Venus Orbiter (2031)
Davinci: https://ssed.gsfc.nasa.gov/davinci/Veritas: https://www.jpl.nasa.gov/missions/veritas/
EnVision: https://www.esa.int/Science_Exploration/Space_Science/Envisi...
That davinci page is pretty cool!
They found that there's very little hydrogen present in its volcanic gases, suggesting that it doesn't have much interior water, and that it didn't in the past, which precludes the formation of surface oceans. It's not incontrovertible proof (something we'll most likely never have), but it's still solid evidence against oceanic life.
It's been a while since I took a solar system geology course but IIRC Venus undergoes periodic planet wide resurfacing events that wipe out the geological record.
I'm not sure if we can tell when those events started happening. It's entirely plausible they only started happening recently (geologically speaking) which possibly would not show up in the gas analysis.
Again, I'm no expert.