man i totally get the frustration with extension tubes... they are fine for static objects but a nightmare for insects that actually move. since you are on a budget the best path is definitely adapting older glass. the Canon EF 100mm f/2.8 Macro USM is the most logical pick here. it is the non-l version but it is still tack sharp and the usm motor is very snappy. you can easily find these used for about 300 dollars and they work perfectly with the ef-rf adapter. another route is the Sigma 105mm f/2.8 EX DG OS HSM Macro which is a legendary lens for the price. it gives you true 1:1 magnification and the working distance is much better for bugs so you dont scare them away. i have found the sigma glass to be very consistent on r series bodies. it is a bit heavy but the optical quality is hard to beat for under 400 bucks. just check the firmware is up to date if you buy an older used copy. you might be tempted by the Canon RF 85mm f/2 Macro IS STM because it is native but be careful. it only offers 0.5x magnification so it is not a true macro lens in the way you probably need for tiny insects. for a project due in two weeks you want that 1:1 capability. honestly just grab the used ef 100mm and call it a day... it is basically the gold standard for budget macro work and wont let you down on focus speed.

yeah adapting older glass is definitely the play. the Tamron SP 90mm f/2.8 Di VC USD Macro F017 is a beast... it wont let you down and holds up great on the r5.

I totally agree with leonnFak about the native glass trade-offs, though honestly, I took a gamble on some manual glass and havent looked back. I was in the same spot after getting my R5—zero cash left for fancy L glass. I ended up grabbing a Laowa 100mm f/2.8 2x Ultra Macro APO Canon EF and it works so well. It is fully manual, which sounds scary, but it is built like a tank and the 2:1 magnification lets you get way closer than those other options. No electronics to fail means no hunting issues like you had with those tubes. If you gotta have AF for those moving bugs tho, I have been really satisfied with the Canon RF 35mm f/1.8 IS Macro STM. It is surprisingly sharp for a budget lens and the combo of lens IS and the R5s IBIS is just killer. Couple quick tips for the R5:

• Turn on focus peaking (red or yellow works best for me) to nail manual shots.
• Use the 1.6x crop mode if you need extra reach without losing focus distance.

tl;dr: check out the Canon RF 35mm f/1.8 Macro IS STM for native af or the Venus Laowa 85mm f/5.6 2x Ultra Macro APO RF for crazy 2:1 reach. honestly i have been so satisfied with the 35mm on my r5... even tho it is only 0.5x magnification, you can crop so much with that sensor that it almost doesnt matter for most stuff. it is super snappy and way more reliable than messing with tubes. i found mine used on mpb for under 400 bucks and it works well for basically everything. if you really need to get microscopic tho the laowa 85mm is a hidden gem. it is tiny and native mount so no adapter needed. definitely peek at keh.com or fred miranda forums for used deals cuz you can save a boatload of money there. people are always swapping gear so you can usually find a mint copy of older macro glass for cheap if you keep an eye out. good luck with the bugs!

Checking in a few hours late here, but wanted to throw in some reliable options that actually fit your budget. If you want to stay native to avoid adapter issues, there are trade-offs but they definitely work.

• Canon RF 85mm f/2 Macro IS STM - it is only a 0.5x macro, but the image stabilization is killer and the AF is predictable.
• Tokina atx-i 100mm f/2.8 FF Macro Canon EF - very solid build and super sharp, tho youll need the EF-RF adapter for it. Ngl, shooting bugs is tough. One quick tip: try using a higher shutter speed or a basic flash setup. Even a cheap flash makes a huge difference in getting those tiny details sharp when the AF is struggling. You got this, the R5 sensor will definitely pick up the detail if the glass is decent enough.