Major props for wanting to take good care of your little fellow!!! I feel like I can give you a little bit more personalized help on the maintenance with MS thing, my mother-in-law to be (hopefully XD) has pretty severe MS, so I kinda know what the limitations are there.
First off, most bettas can live happily in anything from a 1G to XXGs. They're usually not bothered by more space. A 30G should be fine for 1 betta and 1 pleco, you can use this site to calculate stocking for a tank:
That being said, if you're having balance issues, standing to clean that tank for the 15ish mins it's going to take may not be ideal. I would stick with the 1G, or you can drop about $15 American on a 2.5G, which opens up tons of windows for care.
A siphon is designed to not suck up the substrate, just kinda 'air' it out and get the gunk (fancy peeps call it mulm) out. I do have sand in 2 of my tanks, I lose maybe 100 grains every water change? Enough to be like "oh look, there's sand in the bucket" but not enough to make any difference in the tank. A siphon is DEFINITELY your best bet for cleaning. You shouldn't ever have to carry a fish tank, unless you're moving it for some reason (like making room on the table for more <_<).
Regardless of tank size, you do need a heater. I have found it difficult to heat my 1G tanks, the pre-set 8W heaters I have are great in the winter, but are overheating the water in the summer. Again, having a 2.5G means you can fit an adjustable heater in there, which is really idea. Obv, you can fit whatever the heck you want into that big tank.
Filtration is a matter of opinion. I do not have filters in my 2.5G tanks yet (they're in next month's budget, hopefully), so I do 3 weekly water changes. T&Th I do a 50% change, and Sundays I do a 100% change.
With a larger tank, there is no reason not to filter it. I have no experience with filters there, so someone else is gonna have to help you out with that. The issue I could see with that would be moving the discarded water... You would still need to do a significant WC every week, close to 50%... That's potentially 20G of water you're needing to move around. You could do like, 10% every day during the week or something, but that's tedious.
Algae isn't bad, it's just ugly. In a 2.5G with good upkeep, you could get a little snail (No Plecos!!!!) to help munch on the algae. I always always recommend Nerite snails as betta-friends. They're adorable, and great algae eaters, and armored little fellows. No fleshy bits for a betta to nibble on.
There's some more advice needed if you chose to filter, but this is long enough for one post... Please let me know if I can help with anything else!!!